[CMake] Suggest change to FindOpenGL.cmake

[Clifford Yapp]

Currently, the FindOpenGL.cmake file in CMake doesn't have much
support for X11 OpenGL on APPLE - I'd like to suggest a relatively
small change that I've had pretty good luck with and would make this
much simpler for a CMakeLists.txt file to address:

http://public.kitware.com/Bug/view.php?id=11631

This basically supplies an option to allow the calling CMakeLists.txt
file to enable(default) or disable Aqua OpenGL on OSX - so for
situations where an X11 compile is taking place, the parent script
sets the Aqua variable to OFF ahead of time and the script will return
(if found) the X11 OpenGL instead.

It's a pretty small change, and I'd be willing to rework it if
desired, but I'd like to have this or a similar ability by default
since X11 OpenGL builds are quite common for BRL-CAD.

Cheers,
CY