build: Use AM_CFLAGS instead of GCC_CFLAGS everywhere

This will make it easier if we ever want to add new flags to everything
in the future.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>