Avoid syntax errors with variables like GL_3D.
Add support for OS-X.
Alphabetically sort constants.

This will make updates easier.
Add constants as exported from OS-X 10.8.4.

Command to generate was:
cat /System/Library/Frameworks/OpenGL.framework/Versions/A/Headers/gl.h | python constants.py > ../pygl/constants.py
Change to argparse and find the header ourself.

Remove the need for cat'ing in the header if not
specified.