Debian packaging: do not ignore all errors from make distclean

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@2289 d4fa192b-c00b-0410-8231-f00ffab90ce4

debian/rules

@@ -49,7 +49,7 @@ clean:
 	dh_testroot
 	rm -f build-stamp 
 
-	-$(MAKE) distclean
+	[ ! -f Makefile ] || $(MAKE) distclean
 ifneq "$(wildcard /usr/share/misc/config.sub)" ""
 	cp -f /usr/share/misc/config.sub config.sub
 endif