contrib/buildsystems: error out on unknown option

One time too many did this developer call the `generate` script passing
a `--make-out=<PATH>` option that was happily ignored (because there
should be a space, not an equal sign, between `--make-out` and the
path).

And one time too many, this script not only ignored it but did not even
complain. Let's fix that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/buildsystems/engine.pl

@@ -57,6 +57,8 @@ sub showUsage
         open(F, "<$infile") || die "Couldn't open file $infile";
         @makedry = <F>;
         close(F);
+    } else {
+        die "Unknown option: " . $arg;
     }
 }