Run gcc supported flags test with -Werror

- On gcc versions supporting -fstack-protector, not all platforms
  support this feature, causing gcc to emit only a warning instead
  of an error. Which then causes warning-spew on every compiled file.
  Run all the supported flag tests with -Werror to turn the warnings
  into errors, there could be other similar cases besides stack protector.
- Patch originally from Arnaud Patard according to this Mandriva commit:
  http://svn.mandriva.com/cgi-bin/viewvc.cgi/packages?view=revision&revision=448632

configure.ac

@@ -43,7 +43,7 @@ if test "$GCC" = yes; then
     old_cflags=$CFLAGS
     echo
     for flag in $cflags_to_try; do
-        CFLAGS="$CFLAGS $flag"
+        CFLAGS="$CFLAGS $flag -Werror"
         AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[return 0;]])],[
                 echo "   $flag"
                 RPMCFLAGS="$RPMCFLAGS $flag"