Make AC_PROG_GCC_TRADITIONAL a compatibility alias for AC_PROG_CC.

Make AC_PROG_GCC_TRADITIONAL a compatibility alias for AC_PROG_CC.

This macro is one of the last remaining internal uses of AC_EGREP_CPP.
It has only ever done anything useful with GCC, and GCC dropped
support for ‘traditional’ compilation in version 3.3 (released 2003)
so I do not think it is worth trying to preserve.

* lib/autoconf/c.m4 (AC_PROG_GCC_TRADITIONAL): Make into a
  compatibility alias for AC_PROG_CC, similar to AC_PROG_CC_STDC.
* lib/autoconf/general.m4 (AC_EGREP_CPP): Remove stale comment.
* doc/autoconf.texi, NEWS: Document this change.
* tests/mktests.pl: Exclude AC_PROG_GCC_TRADITIONAL from
  autoupdate tests.