GC_VIDEO_XXX Configuration

.. you can now select what kind of video widget you
   want to build.

   GC_VIDEO_NONE       - No media support compiled in
   GC_VIDEO_VLC        - Use VLC (only on Win/Linux)
   GC_VIDEO_QUICKTIME  - Use Apple QuickTime (only on OS X)
   GC_VIDEO_QT5        - QT5 video and associated backends (O/S dependent)

NOTE: This needs testing on non-Linux platforms.