Thanks for the tips! Yes it can always be worked around, however the intuition I developed from doing it for over a hundred of projects [1] is that, with script like that there usually would be more compiler assumptions in C code as well, then I just move on to other projects.
(And it also feels awkward whether to report portability issues to projects with such script because it's like a clear "not for other compilers" message, similar to projects that adopted Meson)
[1] https://github.com/fuhsnn/slimcc/blob/94dde1b8080e02b188ea89...