m4: add Werror when checking for compiler flags

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>