Stagify Gstreamer ports.
Fix build of gstreamer and gstreamer1 with glib 2.38.x .
Update gstreamer and gstreamer-plugins to new rules, USES=gmake pathfix,
new LIB_DEPENDS syntax.
Only pull in flex if base version is too old.
Be more selective for which OSVERSIONS we need gcc due to the clang bug.
Obtained from: Gstreamer upstream