Since this constitutes mostly of macros and type definitions, it would
not fit too well in compat/. Most of the code would end up in
<vlc_fixups.h> rather than compat/. Moreover, I doubt that those
functions would be detected properly with AC_CHECK_FUNCS or
AC_REPLACE_FUNCS.

Anyway, VLC already has a separate header, and it will need to keep it
until <stdatomic.h> can be relied upon... many years from now.