Fix the Configure escape with usecrosscompile but no targethost

Commit 97076f2 added a graceful exit to Configure when targethost is
not defined but usecrosscompile is. However, this is not reached because
there is a similar check a bit earlier that makes Configure bail out.

Make the earlier check warn instead of croaking.

The use case for this combination is supplying an external config.sh
suitable for the target platform, avoiding the need for configuration
probes.

Author: ntyni

Configure

@@ -2973,7 +2973,7 @@ $define|true|[yY]*)
 	*)  echo "Using targetarch $targetarch." >&4 ;;
 	esac
 	case "$targethost" in
-	'') echo "Targethost not defined." >&4; croak=y ;;
+	'') echo "Targethost not defined." >&4; croak=n ;;
 	*)  echo "Using targethost $targethost." >&4
 	esac
 	locincpth=' '