std-gnu23: don’t test stdbit.h Although freestanding C23 compilers must support <stdbit.h>, some current sort-of-C23 environments lack it. We still want to use options like -std=gnu23 on such platforms. Problem reported by Bruno Haible in: https://lists.gnu.org/r/bug-gnulib/2024-11/msg00141.html * m4/std-gnu23.m4 (_AC_C_C23_TEST_GLOBALS): Do not test stdbit.h.