Don't use mutexes if there are no threads.

This commit was SVN r11352.

opal/event/event.c

@@ -461,9 +461,13 @@ opal_event_process_active(struct event_base *base)
 		while (ncalls) {
 			ncalls--;
 			ev->ev_ncalls = ncalls;
+#if OMPI_ENABLE_PROGRESS_THREADS
 			opal_mutex_unlock(&opal_event_lock);
 			(*ev->ev_callback)((int)ev->ev_fd, ev->ev_res, ev->ev_arg);
 			opal_mutex_lock(&opal_event_lock);
+#else
+			(*ev->ev_callback)((int)ev->ev_fd, ev->ev_res, ev->ev_arg);
+#endif  /* OMPI_ENABLE_PROGRESS_THREADS */
 		}
 	}
 #endif