compiler-flags: Add an optional second argument for custom warnings

This allows module authors to choose to enforce stricter warnings on
a per-module basis, to avoid the situation where an outside
contributor submits a patch which introduces warnings the maintainer
has enabled locally.

Closes: https://bugzilla.gnome.org/show_bug.cgi?id=707475
compiler-flags: Add a #serial line

As suggested in bug #707475, this will ensure the latest version of
gnome-compiler-flags.m4 is used if conflicting versions are found in
different directories in the autoconf search path.

Helps: https://bugzilla.gnome.org/show_bug.cgi?id=707475