glx: Fix GLX_CONTEXT_RELEASE_BEHAVIOR_ARB handling

None of this was getting compiled because we hadn't defined the macro
(and aren't getting them from <GL/glxext.h> because reasons). Fix that.

Fixes: xorg/xserver#684
3 jobs for fix-glx-context-release-behaviour in 8 minutes and 48 seconds (queued for 8 minutes and 27 seconds)