OpenGL support for Mac OS X:

"./configure --enable-hopengl" on Mac OS X now uses the OpenGL libs that come
with Apple's "Quartz" display system.
"./configure --enable-hopengl=x11" looks for an X11 implementation of OpenGL
(which is an optional install on Mac OS X).

Added a corresponding "USE_QUARTZ_OPENGL" flag to config.h