perl: fix make clean

When perl/Makefile is stale with respect to perl/Makefile.PL, it
prevents "make clean" from completing which is quite irritating.
Fix it by calling subdirectory make clean twice as needed.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -744,7 +744,7 @@ clean:
 	rm -f $(GIT_TARNAME).tar.gz git-core_$(GIT_VERSION)-*.tar.gz
 	rm -f $(htmldocs).tar.gz $(manpages).tar.gz
 	$(MAKE) -C Documentation/ clean
-	[ ! -e perl/Makefile ] || $(MAKE) -C perl/ clean
+	[ ! -e perl/Makefile ] || $(MAKE) -C perl/ clean || $(MAKE) -C perl/ clean
 	$(MAKE) -C templates/ clean
 	$(MAKE) -C t/ clean
 	rm -f GIT-VERSION-FILE GIT-CFLAGS