starsnet Yepp, I forgot about that: if you try to do so, it'll say that gcc has already been configured even if you remove the automatically detected ones. Nevertheless, even if it complains that clang is selected for the kit, it does use gcc (due to mkspecs) and does the job. But I agree it's not nice. I'll check if it can be done better. Actually, I'm just thinking of writing an mkspec file for clang as it's basically about setting a bunch of relevant shell variables like CC.
UPDATE: Well, no clue why, yesterday it did not let me add custom compilers for /usr/bin/gcc and /usr/bin/c++ but today it does. That makes the two warnings (about different compilers being set for the kit than in mkspecs) vanish.