GL/gl.h provides some definitions (GL_FALSE, GL_ONE, etc) that have
the same value as other gl headers but are represented differently
(0 vs 0x0 and 1 vs 0x1).
This causes compiler warnings about redefining such definitions when
including GL/gl.h with other gl headers.
Fixes http://bugs.freedesktop.org/show_bug.cgi?id=57802
Signed-off-by: Brian Paul <brianp@vmware.com>