Move the -Wtraditional warning from the global cpp flags to when the -p

(portability warnings) switch is used.  Add -Wno-system-headers after it so
that we dont get 500 screenfulls of warnings about #elif in /usr/include.
I'm not entirely happy with this.  Maybe cdefs.h shouldn't use #elif and
instead nest #else clauses?

usr.bin/xlint/xlint/xlint.c

@@ -351,8 +351,6 @@ main(int argc, char *argv[])
 
 	appdef(&cflags, "lint");
 
-	appcstrg(&lcflags, "-Wtraditional");
-
 	appcstrg(&deflibs, "c");
 
 	if (signal(SIGHUP, terminate) == SIG_IGN)
@@ -404,6 +402,7 @@ main(int argc, char *argv[])
 			break;
 
 		case 'p':
+			appcstrg(&lcflags, "-Wtraditional -Wno-system-headers");
 			appcstrg(&l1flags, "-p");
 			appcstrg(&l2flags, "-p");
 			if (*deflibs != NULL) {