C11 provides static assertions via _Static_assert and
provides static_assert as a convenience define for this
in assert.h. Our codebase uses the latter, as _Static_assert
has actually already been deprecated in C23.

Not all toolchains that declare support for C11 actually
support it; e.g. MSVC 19.27 does not support _Static_assert,
but somehow supports static_assert. MSVC 19.27 admits to be
a "preview implementation of the ISO C11 standard",
so this is not surprising (MSVC 19.28 does not come with
this caveat).

Furthermore some FATE boxes [1] use old GCC toolchains (with
only experimental support for C11) where _Static_assert is
supported, but assert.h does not provide the fallback define.
They are broken since the first usage of static_assert.

This commit therefore checks whether static_assert and
_Static_assert work with assert.h included; if not,
configure errors out.

This intentionally drops support for MSVC 19.27. Users like
the old FATE boxes above can still add -Dstatic_assert=_Static_assert
to cflags as a workaround if desired.

[1]: https://fate.ffmpeg.org/report.cgi?time=20240321123620&slot=sh4-debian-qemu-gcc-4.7Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>