gl/meson: Allow for the use of libGL as well as OpenGL

OpenGL.framework and libGL are two different providers of a GL
implementation that can be linked into an application together.
Status Job ID Name Coverage
  Preparation
passed #52884
manifest

00:00:25

canceled #52883
manifest

failed #52781
manifest

 
  Build
passed #52782
build fedora x86_64

00:03:45

 
  Full Builds
passed #52783
build android arm64 api28

00:03:21