This is to prevent bogus QA warnings like the following in GStreamer ports
Using USE_XORG alone is deprecated, please use USES=xorg
gstreamer1?-plugins/Makefile.common can under some circumstances create empty USE_XORG, USE_GNOME, etc. which trigger the warnings.
gstreamer-plugins-good triggers the USE_XORG warning because USES is overwritten again after including the master Makefile.
gstreamer-plugin-sdl was missing USES=sdl.