Makefile: have perl scripts depend on NO_PERL setting

If NO_PERL is not set, our perl scripts are built as usual. If it is set, then we build "dummy" versions that tell you git was built without perl support and exit gracefully. However, if you switch to NO_PERL in a directory with existing build artifacts, we do not notice that the files need rebuilt. We see only that they are newer than the "unimplemented.sh" wrapper and assume they are done. So doing: make make NO_PERL=Nope would result in a git-add--interactive script that uses perl (and running the test suite would make use of it). Instead, we should trigger a rebuild of the perl scripts anytime NO_PERL changes. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>