x11/picom is currently v9.1 released on Feb 13 2022 and updated in pkgsrc on Sep 4 2022.
Currently, picom is at v11.2 released on Feb 13 2024, a package for this version is available as wip/picom.

All versions of picom after v9.1 require MesaLib >= 21 and need to be built with graphics/MesaLib from pkgsrc, as the base system uses MesaLib 19.

~> uname -rsv
NetBSD 10.99.10 NetBSD 10.99.10 (GENERIC) #0: Mon Apr 22 03:38:49 UTC 2024  mkrepro@mkrepro.NetBSD.org:/usr/src/sys/arch/amd64/compile/GENERIC
~> glxinfo -B
name of display: :0
display: :0  screen: 0
direct rendering: Yes
Extended renderer info (GLX_MESA_query_renderer):
    Vendor: Intel Open Source Technology Center (0x8086)
    Device: Mesa DRI Intel(R) Haswell Mobile  (0xa16)
    Version: 19.1.17
    Accelerated: yes
    Video memory: 1536MB
    Unified memory: yes
    Preferred profile: core (0x1)
    Max core profile version: 4.5
    Max compat profile version: 3.0
    Max GLES1 profile version: 1.1
    Max GLES[23] profile version: 3.1
OpenGL vendor string: Intel Open Source Technology Center
OpenGL renderer string: Mesa DRI Intel(R) Haswell Mobile
OpenGL core profile version string: 4.5 (Core Profile) Mesa 19.1.17
OpenGL core profile shading language version string: 4.50
OpenGL core profile context flags: (none)
OpenGL core profile profile mask: core profile

OpenGL version string: 3.0 Mesa 19.1.17
OpenGL shading language version string: 1.30
OpenGL context flags: (none)

OpenGL ES profile version string: OpenGL ES 3.1 Mesa 19.1.17
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 3.10

Although, the base system also contains MesaLib 21, this is not on by default.
After some discussion with maya@ and wiz@, thank you both, I've decided to build a custom install image with MesaLib 21 enabled.

Upgrading the system using this custom image should allow me to build wip/picom without the need to install graphics/MesaLib from pkgsrc.

As a bonus, this should also avoids the need to have multiple LLVM versions installed, as graphics/MesaLib requires LLVM-13 and I already have 17 installed.

We need to fetch the sources, only src and xsrc are needed, and create the image using build.sh.
Note: approx. 32 Gb of free disk space are required.

~> ./build.sh -h
Usage: build.sh [-EnoPRrUux] [-a ARCH] [-B BID] [-C EXTRAS]
                [-c COMPILER] [-D DEST] [-j NJOB] [-M MOBJ] [-m MACH]
                [-N NOISY] [-O OOBJ] [-R RELEASE] [-S SEED] [-T TOOLS]
                [-V VAR=[VALUE]] [-w WRAPPER] [-X X11SRC]
                [-Z VAR]
                OPERATION ...
       build.sh ( -h | -? )

 Build OPERATIONs (all imply "obj" and "tools"):
    build               Run "make build".
    distribution        Run "make distribution" (includes DESTDIR/etc/ files).
    release             Run "make release" (includes kernels & distrib media).

 Other OPERATIONs:
    help                Show this help message, and exit.
    makewrapper         Create nbmake-${MACHINE} wrapper and nbmake.
                        Always performed.
    cleandir            Run "make cleandir".  [Default unless -u is used]
    dtb                 Build devicetree blobs.
    obj                 Run "make obj".  [Default unless -o is used]
    tools               Build and install tools.
    install=IDIR        Run "make installworld" to IDIR to install all sets
                        except 'etc'.  Useful after "distribution" or "release".
    kernel=CONF         Build kernel with config file CONF.
    kernel.gdb=CONF     Build kernel (including netbsd.gdb) with config
                        file CONF.
    releasekernel=CONF  Install kernel built by kernel=CONF to RELEASEDIR.
    kernels             Build all kernels.
    installmodules=IDIR Run "make installmodules" to IDIR to install all
                        kernel modules.
    modules             Build kernel modules.
    rumptest            Do a linktest for rump (for developers).
    sets                Create binary sets in
                        RELEASEDIR/RELEASEMACHINEDIR/binary/sets.
                        DESTDIR should be populated beforehand.
    distsets            Same as "distribution sets".
    sourcesets          Create source sets in RELEASEDIR/source/sets.
    syspkgs             Create syspkgs in
                        RELEASEDIR/RELEASEMACHINEDIR/binary/syspkgs.
    iso-image           Create CD-ROM image in RELEASEDIR/images.
    iso-image-source    Create CD-ROM image with source in RELEASEDIR/images.
    live-image          Create bootable live image in
                        RELEASEDIR/RELEASEMACHINEDIR/installation/liveimage.
    install-image       Create bootable installation image in
                        RELEASEDIR/RELEASEMACHINEDIR/installation/installimage.
    disk-image=TARGET   Create bootable disk image in
                        RELEASEDIR/RELEASEMACHINEDIR/binary/gzimg/TARGET.img.gz.
    params              Create params file with various make(1) parameters.
    show-params         Show various make(1) parameters.
    list-arch           Show a list of valid MACHINE/MACHINE_ARCH values,
                        and exit.  The list may be narrowed by passing glob
                        patterns or exact values in MACHINE or MACHINE_ARCH.
    mkrepro-timestamp   Show the latest source timestamp used for reproducible
                        builds and exit.  Requires -P or -V MKREPRO=yes.

 Options:
    -a ARCH        Set MACHINE_ARCH=ARCH.  [Default: deduced from MACHINE]
    -B BID         Set BUILDID=BID.
    -C EXTRAS      Append EXTRAS to CDEXTRA for inclusion on CD-ROM.
    -c COMPILER    Select compiler from COMPILER:
                       clang
                       gcc
                   [Default: gcc]
    -D DEST        Set DESTDIR=DEST.  [Default: destdir.${MACHINE}]
    -E             Set "expert" mode; disables various safety checks.
                   Should not be used without expert knowledge of the build
                   system.
    -h             Show this help message, and exit.
    -j NJOB        Run up to NJOB jobs in parallel; see make(1) -j.
    -M MOBJ        Set obj root directory to MOBJ; sets MAKEOBJDIRPREFIX=MOBJ,
                   unsets MAKEOBJDIR.
    -m MACH        Set MACHINE=MACH.  Some MACH values are actually
                   aliases that set MACHINE/MACHINE_ARCH pairs.
                   [Default: deduced from the host system if the host
                   OS is NetBSD]
    -N NOISY       Set the noisiness (MAKEVERBOSE) level of the build to NOISY:
                       0   Minimal output ("quiet").
                       1   Describe what is occurring.
                       2   Describe what is occurring and echo the actual
                           command.
                       3   Ignore the effect of the "@" prefix in make
                           commands.
                       4   Trace shell commands using the shell's -x flag.
                   [Default: 2]
    -n             Show commands that would be executed, but do not execute
                   them.
    -O OOBJ        Set obj root directory to OOBJ; sets a MAKEOBJDIR pattern
                   using OOBJ, unsets MAKEOBJDIRPREFIX.
    -o             Set MKOBJDIRS=no; do not create objdirs at start of build.
    -P             Set MKREPRO and MKREPRO_TIMESTAMP to the latest source
                   CVS timestamp for reproducible builds.
    -R RELEASE     Set RELEASEDIR=RELEASE.  [Default: releasedir]
    -r             Remove contents of TOOLDIR and DESTDIR before building.
    -S SEED        Set BUILDSEED=SEED.  [Default: NetBSD-majorversion]
    -T TOOLS       Set TOOLDIR=TOOLS.  If unset, and TOOLDIR is not set
                   in the environment, nbmake will be (re)built
                   unconditionally.
    -U             Set MKUNPRIVED=yes; build without requiring root privileges,
                   install from an unprivileged build with proper file
                   permissions.
    -u             Set MKUPDATE=yes; do not run "make cleandir" first.
                   Without this, everything is rebuilt, including the tools.
    -V VAR=[VALUE] Set variable VAR=VALUE.
    -w WRAPPER     Create nbmake script as WRAPPER.
                   [Default: ${TOOLDIR}/bin/nbmake-${MACHINE}]
    -X X11SRC      Set X11SRCDIR=X11SRC.  [Default: /usr/xsrc]
    -x             Set MKX11=yes; build X11 from X11SRCDIR.
    -Z VAR         Unset ("zap") variable VAR.
    -?             Show this help message, and exit.

The following steps can be done with one command only but, I was curious about the time it takes for each step.
NOTE: The build should be done in unprivileged mode, else creating the install image will fail.

Without any further due ...

~> cd /usr/src
~> ./build.sh -u -U -j4 -O ~/obj tools
[...]
===> Summary of results:
	 build.sh command:    ./build.sh -u -U -j4 -O /home/pin/obj tools
	 build.sh started:    Thu Apr 25 15:21:18 CEST 2024
	 NetBSD version:      10.99.10
	 MACHINE:             amd64
	 MACHINE_ARCH:        x86_64
	 Build platform:      NetBSD 10.99.10 amd64
	 HOST_SH:             /bin/sh
	 No $TOOLDIR/bin/nbmake, needs building.
	 Bootstrapping nbmake
	 share/mk MAKECONF:   /etc/mk.conf
	 MAKECONF file:       /etc/mk.conf
	 TOOLDIR path:        /home/pin/obj/tooldir.NetBSD-10.99.10-amd64
	 DESTDIR path:        /home/pin/obj/destdir.amd64
	 RELEASEDIR path:     /home/pin/obj/releasedir
	 Created /home/pin/obj/tooldir.NetBSD-10.99.10-amd64/bin/nbmake
	 Updated makewrapper: /home/pin/obj/tooldir.NetBSD-10.99.10-amd64/bin/nbmake-amd64
	 Tools built to /home/pin/obj/tooldir.NetBSD-10.99.10-amd64
	 build.sh ended:      Thu Apr 25 15:49:31 CEST 2024

~> ./build.sh -u -U -j4 -V HAVE_MESA_VER=21 -x -O ~/obj release
[...]
===> Summary of results:
	 build.sh command:    ./build.sh -u -U -j4 -V HAVE_MESA_VER=21 -x -O /home/pin/obj release
	 build.sh started:    Thu Apr 25 15:52:10 CEST 2024
	 NetBSD version:      10.99.10
	 MACHINE:             amd64
	 MACHINE_ARCH:        x86_64
	 Build platform:      NetBSD 10.99.10 amd64
	 HOST_SH:             /bin/sh
	 share/mk MAKECONF:   /etc/mk.conf
	 MAKECONF file:       /etc/mk.conf
	 TOOLDIR path:        /home/pin/obj/tooldir.NetBSD-10.99.10-amd64
	 DESTDIR path:        /home/pin/obj/destdir.amd64
	 RELEASEDIR path:     /home/pin/obj/releasedir
	 Updated makewrapper: /home/pin/obj/tooldir.NetBSD-10.99.10-amd64/bin/nbmake-amd64
	 Successful make release
	 build.sh ended:      Thu Apr 25 22:57:38 CEST 2024

~> ./build.sh -u -U -j4 -V HAVE_MESA_VER=21 -x -O ~/obj install-image
[...]
===> Summary of results:
	 build.sh command:    ./build.sh -u -U -j4 -V HAVE_MESA_VER=21 -x -O /home/pin/obj install-image
	 build.sh started:    Fri Apr 26 05:02:44 CEST 2024
	 NetBSD version:      10.99.10
	 MACHINE:             amd64
	 MACHINE_ARCH:        x86_64
	 Build platform:      NetBSD 10.99.10 amd64
	 HOST_SH:             /bin/sh
	 share/mk MAKECONF:   /etc/mk.conf
	 MAKECONF file:       /etc/mk.conf
	 TOOLDIR path:        /home/pin/obj/tooldir.NetBSD-10.99.10-amd64
	 DESTDIR path:        /home/pin/obj/destdir.amd64
	 RELEASEDIR path:     /home/pin/obj/releasedir
	 Updated makewrapper: /home/pin/obj/tooldir.NetBSD-10.99.10-amd64/bin/nbmake-amd64
	 Successful make install-image
	 build.sh ended:      Fri Apr 26 05:23:55 CEST 2024

We should now have the desired install images in obj/releasedir/images.

~> ls /home/pin/obj/releasedir/images/
.rw-r--r-- pin wheel 161 B  Fri Apr 26 05:23:41 2024  MD5
.r--r--r-- pin wheel 426 MB Fri Apr 26 05:23:28 2024  NetBSD-10.99.10-amd64-bios-install.img.gz
.r--r--r-- pin wheel 426 MB Fri Apr 26 05:13:03 2024  NetBSD-10.99.10-amd64-install.img.gz
.rw-r--r-- pin wheel 359 B  Fri Apr 26 05:23:55 2024  SHA512

As we built release above, a manual upgrade is also possible but, let's unpack the newly built image, copy it to an usb and use sysinst.

~> gunzip NetBSD-10.99.10-amd64-install.img.gz
~> su
Password:
# dd if=/dev/zero of=/dev/rsd0d bs=1m count=1m
# dd if=NetBSD-10.99.10-amd64-install.img ibs=1m | progress dd of=/dev/rsd0d obs=1m
2493 MiB    6.52 MiB/s 2500+0 records in
5120000+0 records out
2621440000 bytes transferred in 382.641 secs (6850912 bytes/sec)
5120000+0 records in
2500+0 records out
2621440000 bytes transferred in 382.721 secs (6849480 bytes/sec)
  2500 MiB    6.53 MiB/s
# exit

Boot from the newly created install media and perform an upgrade of the system.

~> uname -rsv
NetBSD 10.99.10 NetBSD 10.99.10 (GENERIC) #0: Thu Apr 25 22:39:14 CEST 2024  pin@mybox:/home/pin/obj/sys/arch/amd64/compile/GENERIC
~> glxinfo -B
name of display: :0
display: :0  screen: 0
direct rendering: Yes
Extended renderer info (GLX_MESA_query_renderer):
    Vendor: Intel Open Source Technology Center (0x8086)
    Device: Mesa DRI Intel(R) HD Graphics 4400 (HSW GT2) (0xa16)
    Version: 21.3.7
    Accelerated: yes
    Video memory: 1536MB
    Unified memory: yes
    Preferred profile: core (0x1)
    Max core profile version: 4.5
    Max compat profile version: 3.0
    Max GLES1 profile version: 1.1
    Max GLES[23] profile version: 3.1
OpenGL vendor string: Intel Open Source Technology Center
OpenGL renderer string: Mesa DRI Intel(R) HD Graphics 4400 (HSW GT2)
OpenGL core profile version string: 4.5 (Core Profile) Mesa 21.3.7
OpenGL core profile shading language version string: 4.50
OpenGL core profile context flags: (none)
OpenGL core profile profile mask: core profile

OpenGL version string: 3.0 Mesa 21.3.7
OpenGL shading language version string: 1.30
OpenGL context flags: (none)

OpenGL ES profile version string: OpenGL ES 3.1 Mesa 21.3.7
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 3.10

And yes, I only have one version of MesaLib and LLVM installed 😊