fix build for certain picky versions of gcc

ghc/rts/Task.c

@@ -171,18 +171,18 @@ boundTaskExiting (Task *task)
     IF_DEBUG(scheduler,sched_belch("task exiting"));
 }
 
+#ifdef THREADED_RTS
+#define TASK_ID(t) (t)->id
+#else
+#define TASK_ID(t) (t)
+#endif
+
 void
 discardTask (Task *task)
 {
     ASSERT_LOCK_HELD(&sched_mutex);
     if (!task->stopped) {
-	IF_DEBUG(scheduler,sched_belch("discarding task %p",
-#ifdef THREADED_RTS
-				       (void *)task->id
-#else
-				       (void *)task
-#endif
-		     ));
+	IF_DEBUG(scheduler,sched_belch("discarding task %p", TASK_ID(task)));
 	task->cap = NULL;
 	task->tso = NULL;
 	task->stopped = rtsTrue;