Fix compilation on very old mesa versions. vlj, please check.

src/graphics/glwrap.cpp

@@ -121,9 +121,11 @@ CALLBACK
 debugCallback(GLenum source, GLenum type, GLuint id, GLenum severity, GLsizei length,
               const GLchar* msg, const void *userparam)
 {
+#ifdef GL_DEBUG_SEVERITY_NOTIFICATION
     // ignore minor notifications sent by some drivers (notably the nvidia one)
     if (severity == GL_DEBUG_SEVERITY_NOTIFICATION)
         return;
+#endif
 
     switch(source)
     {