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)
Status Job ID Name Coverage
  Docker Image
passed #273448
debian-testing

00:00:33

 
  Build And Test
passed #273449
autotools-build-and-test

00:06:41

passed #273450
meson-build-and-test

00:01:32