compiler-flags: allow suppressing warnings

Some projects want to see no warnings at all, for example, projects that
are written in Vala. But GNOME_COMPILE_WARNINGS([no]) is a no-op that
does nothing to turn off the default compiler warnings. Presumably if
you ask for no compiler warnings, you want no compiler warnings, so
suppress them in this case instead.

https://bugzilla.gnome.org/show_bug.cgi?id=734973