sync/waitgroup: force sleeping for 1ms

fixes issues #2874 and #2961

examples/news_fetcher.v

@@ -67,7 +67,7 @@ fn main() {
 	
 	wg := sync.new_waitgroup()
 	mtx := sync.new_mutex()
-	mut fetcher := &Fetcher{ids: ids}
+	mut fetcher := &Fetcher{ids: ids mu: 0 wg: 0}
 	fetcher.mu = &mtx
 	fetcher.wg = &wg
 	fetcher.wg.add(ids.len)

vlib/sync/waitgroup.v

@@ -32,7 +32,12 @@ pub fn (wg mut WaitGroup) done() {
 
 pub fn (wg mut WaitGroup) wait() {
 	for wg.active > 0 {
-		// waiting
+		// Do not remove this, busy empty loops are optimized
+		// with -prod by some compilers, see issue #2874
+		$if windows {
+			C.Sleep(1)
+		} $else {
+			C.usleep(1000)
+		}
 	}
 }
-