Updated CMake files to support Cocoa on OSX

A flag OCE_OSX_USE_COCOA was added, enabled by default. This
makes OCE use native Cocoa OpenGl implementation as well as
the Tcl/Tk Cocoa provided with OSX/XCode.

This flag should be set to OFF if ever OSX users still need X11 backend.
Modified headers/source files to handle the MACOSX_USE_GLX option
Ugly hack to remove X compilation issues