Fix autoupdate+AC_PROG_GCC_TRADITIONAL

Problem reported by Josh Straub in:
https://lists.gnu.org/r/bug-autoconf/2026-09/msg00000.html
* lib/autoconf/c.m4 (AC_PROG_CC_C89, AC_PROG_CC_C99)
(AC_PROG_CC_STDC, AC_PROG_GCC_TRADITIONAL):
Invoke AC_PROG_CC directly, instead of using AC_REQUIRE.