util: Do not fail to build on unknown pthread_setname_np

This is only used for debugging, so better making porting on various systems
less hard.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3229>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3229>

src/util/u_thread.h

@@ -84,7 +84,7 @@ static inline void u_thread_setname( const char *name )
 #elif DETECT_OS_HAIKU
    rename_thread(find_thread(NULL), name);
 #else
-#error Not sure how to call pthread_setname_np
+#warning Not sure how to call pthread_setname_np
 #endif
 #endif
    (void)name;