This merges in changes that were made to Autoconf master
in April 2024. It causes ‘configure’ to prefer the C compiler to
enable C23 features if possible; otherwise, ‘configure’
behaves as before, using C11 if available and C99 if not.
* admin/merge-gnulib (GNULIB_MODULES): Add std-gnu23.
(AVOIDED_MODULES): Add (i.e., start avoiding) std-gnu11.
* m4/std-gnu11.m4: Remove.
* m4/std-gnu23.m4: Add.
