And add comment explaining why we don't set _POSIX_C_SOURCE

Fixes: libass#840

To be used in CI, to verify we don't end up making something non-standard required by accident

This matches autotools and some of our outline related code
is technically invalid in ISO C99 due to passing a “double[3][3]”
to a function expecting a “const double[3][3]”.
This became officially allowed with ISO C23 and was a GNU
extensions for much longer making compilers unlikely to choke on it,
but for now retain gnu99.

MSVC ofc doesn’t support gnu99, but meson will ignore the option and
in fact already ignored c99 for MSVC only honouring c11, c17 and c18.
mesonbuild/meson#7611