Check for some header defines before redefining functions. Silences warnings.

src/mesa/drivers/dri/common/dri_util.h

@@ -558,8 +558,11 @@ extern float
 driCalculateSwapUsage( __DRIdrawablePrivate *dPriv,
 		       int64_t last_swap_ust, int64_t current_ust );
 
+/* Test for the GLX header glx.h */
+#ifndef GLX
 extern void 
 (*glXGetProcAddress(const GLubyte *procname))( void );
+#endif
 
 #endif /* GLX_DIRECT_RENDERING */