Change USE_GL to become USES=gl.
Change USE_GL to become USES=gl. You still need to specify USE_GL as well,
to specify which gl components you need, such as egl glesv2 glut glu glw gl.
Only specifying USE_GL is supported for now, to ease in transition. It is
however deprecated, and will be removed eventually.
Specifying USES=gl without USE_GL is an error.
USE_GL=yes is also deprecated, but supported for now. It implies USE_GL=glu.
Fix fallout from this change.