Renable compiler warnings and fix a few that show up

The switch to GNOME_COMPILE_WARNINGS() caused -Wall and other
warnings to not actually be used since GNOME_COMPILE_WARNINGS()
just sets WARN_CFLAGS. Add WARN_CFLAGS to AM_CFLAGS so that
it takes effect.

Add  -Wno-error=deprecated-declarations so that when -Werror is
enabled, we don't fail on all the deprecated cogl and clutter
symbols.

https://bugzilla.gnome.org/show_bug.cgi?id=730408