Add config detection for JEMALLOC_HAVE_PTHREAD_SET_NAME_NP.

and use it on the background thread name setting.

src/background_thread.c

@@ -467,7 +467,7 @@ background_thread_entry(void *ind_arg) {
 	assert(thread_ind < max_background_threads);
 #ifdef JEMALLOC_HAVE_PTHREAD_SETNAME_NP
 	pthread_setname_np(pthread_self(), "jemalloc_bg_thd");
-#elif defined(__FreeBSD__) || defined(__DragonFly__)
+#elif defined(JEMALLOC_HAVE_PTHREAD_SET_NAME_NP)
 	pthread_set_name_np(pthread_self(), "jemalloc_bg_thd");
 #endif
 	if (opt_percpu_arena != percpu_arena_disabled) {