pin What did you do to fix it manually?
make > cmd, edited cmd to add libtextosaurus.so after the -o option and ran
sh cmd. It produces libtextosaurus.so instead of a.out, that's all. It doesn't fix the next issue, which is that
make install doesn't copy libtextosaurus.so to /usr/pkg/lib.
Furthermore, I may be wrong, but I think there should be a .a or .sa file to copy along with the .so and it is not produced.
Finally, when installing libraries, there is usually a symlink to the latest version in the lib directory, i.e. the .so file name should be libtextosaurus-0.9.11.so and there should be a libtextosaurus.so symlink in the same directory pointing at libtextosaurus-0.9.11.
This tends to indicate that qmake produces an incorrect Makefile. I guess qmake has already been used to build other Qt applications on NetBSD, so we can assume it is correct. The only remaining possibility is that qmake invocation for this application must be different on NetBSD, either by supplying additional command line arguments, or by tweaking the .pro files. 🙁
BTW, re-reading build.pro, I've realized I should have cloned the repo with the --recurse-submodules option. However, it doesn't solve the linking problem.
I've also seen that textosaurus requires C++17 but gcc implements only C++14, so I installed gcc10 and gcc10-libs and modified my PATH to use it, but it doesn't seem to be sufficient to get C++17 support.