Fails to build properly with autoconf 2.72.90 (2.73 pre-release)

If I build gnutls with autoconf 2.72.90 (the 2.73 pre-release) then the compiler is mis-detected:

WARNING: Compiler does not support C99. It may not be able to compile the project.

This is GCC 15.2, it most certainly does support C99. However, the configure code in gnutls/configure.ac. responsible for this check is quite broken:

AS_CASE([$ac_prog_cc_stdc],
  [c11 | c99], [AC_DEFINE([C99_MACROS], 1, [C99 macros are supported])],
  [AC_MSG_WARN([[Compiler does not support C99. It may not be able to compile the project.]])]
)

This is because ac_prog_cc_stdc is now c23, due to this change in autoconf:

*** AC_PROG_CC now prefers C23 if available.

This is trivially fixed by adding c23 to the list of available standards, but the test is very fragile. Does gnutls even need to cater for non-C99 compilers? That's a pretty niche market these days, surely.