-W option, similar to -w, but makes warnings into errors,
so that large builds that should be warning-free will stop in their tracks.

Reference: /n/sources/patch/applied/cc-warn2errs-2
Date: Mon Mar  4 09:48:53 CET 2013
Signed-off-by: charles.forsyth@gmail.com

--- /sys/src/cmd/cc/sub.c	Mon Mar  4 09:47:23 2013
+++ /sys/src/cmd/cc/sub.c	Mon Mar  4 09:47:18 2013
@@ -1187,12 +1187,15 @@
 	char buf[STRINGSZ];
 	va_list arg;
 
-	if(debug['w']) {
-		Bprint(&diagbuf, "warning: ");
+	if(debug['w'] || debug['W']) {
 		va_start(arg, fmt);
 		vseprint(buf, buf+sizeof(buf), fmt, arg);
 		va_end(arg);
-		Bprint(&diagbuf, "%L %s\n", (n==Z)? nearln: n->lineno, buf);
+		if(debug['W']) {
+			diag(n, "%s", buf);
+			return;
+		}
+		Bprint(&diagbuf, "warning: %L %s\n", (n==Z)? nearln: n->lineno, buf);
 
 		if(n != Z)
 		if(debug['v'])