Bug fix #91: Using pkg-config to find drm.h & OpenGL changes

- #91: Use pkg-config to find drm.h to avoid issues on FreeBSD. Thanks
  to hun7err for pointing out and providing patch.

- #89: Add default shadow exclusion rule for notify-osd. Thanks to
  DanielRS.

- Check for abundant positional arguments.

- Use paint target window (root window / overlay window) instead of
  ps->reg_win to create GLXContext. (May have negative effects on OpenGL
  VSync.) Add new OpenGL helpers functions, to prepare for the new
  OpenGL backend.

- Dump more info of a PropertyNotify with DEBUG_EVENTS.