mesa: Rename GLX_USE_TLS to USE_ELF_TLS.

These days it is not GLX only and it does not work with all TLS
implementations.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

src/mapi/mapi_glapi.c

@@ -38,11 +38,11 @@
  * u_current.c.
  */
 
-#ifdef GLX_USE_TLS
+#ifdef USE_ELF_TLS
 /* not used, but defined for compatibility */
 const struct _glapi_table *_glapi_Dispatch;
 const void *_glapi_Context;
-#endif /* GLX_USE_TLS */
+#endif /* USE_ELF_TLS */
 
 void
 _glapi_destroy_multithread(void)