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/loader/loader.c

@@ -552,7 +552,7 @@ loader_open_driver(const char *driver_name,
          next = end;
 
       len = next - p;
-#if GLX_USE_TLS
+#if USE_ELF_TLS
       snprintf(path, sizeof(path), "%.*s/tls/%s_dri.so", len, p, driver_name);
       driver = dlopen(path, RTLD_NOW | RTLD_GLOBAL);
 #endif