m4: add Werror when checking for compiler flags

Seemingly that at some point clang started accepting _any_ flags,
whereas previously it would error out.

These days, you can give it -Whamsandwich and it will succeed, while
at the same time throwing an annoying warning.

Add -Werror so that everything gets flagged and set accordingly.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108082
Cc: Vinson Lee <vlee@freedesktop.org>
Repored-by: Vinson Lee <vlee@freedesktop.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

Signed-off-by: Yury V. Zaytsev <yury@shurup.com>
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

m4.include/ax_check_compile_flag.m4

@@ -34,6 +34,11 @@
 #   and this notice are preserved.  This file is offered as-is, without any
 #   warranty.
 
+# Yury Zaytsev:
+# Toggle Werror since at some point clang started treating unknown -W
+# flags as warnings, succeeding with the build, yet issuing an annoying
+# warning.
+
 #serial 6
 
 AC_DEFUN([AX_CHECK_COMPILE_FLAG],
@@ -41,7 +46,7 @@ AC_DEFUN([AX_CHECK_COMPILE_FLAG],
 AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
 AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
   ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
-  _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1"
+  _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1 -Werror"
   AC_COMPILE_IFELSE([m4_default([$5],[AC_LANG_PROGRAM()])],
     [AS_VAR_SET(CACHEVAR,[yes])],
     [AS_VAR_SET(CACHEVAR,[no])])

m4.include/mc-cflags.m4

@@ -13,18 +13,6 @@ AC_DEFUN([mc_CHECK_CFLAGS],[
 
     mc_configured_cflags=""
 
-dnl    AC_MSG_CHECKING([CC is $CC])
-
-    dnl https://stackoverflow.com/questions/52557417/how-to-check-support-compile-flag-in-autoconf-for-clang
-    case "$CC" in
-    clang*)
-        EXTRA_OPTION="-Werror"
-        ;;
-    *)
-        EXTRA_OPTION=""
-        ;;
-    esac
-
     dnl Sorted -f options:
     case "$CC" in
     gcc*)