AC_C_CONST: don't reject gcc -Werror -Wall

AC_C_CONST: don't reject gcc -Werror -Wall

* lib/autoconf/c.m4 (AC_C_CONST): Don't reject gcc when it is used
with -Werror -Wall during configuring.  It's unwise to use GCC
that way, but apparently enough people do it nowadays that it's an
issue.  These days nobody uses the old compilers that the old
tests reject, so we can't test this fix against them, but it's
more important to work with modern GCC (even when misused) than to
work with no-longer-used compilers.  Problem reported by Shevek in
<http://lists.gnu.org/archive/html/bug-autoconf/2008-11/msg00007.html>
and raised again by Dan Kegel in
<http://lists.gnu.org/archive/html/bug-autoconf/2011-08/msg00020.html>.