Makefile: ignore perl/ subdirectory under NO_PERL

The install target still descends into perl subdirectory when NO_PERL is
requested.  Fix this.

Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1529,7 +1529,9 @@ install: all
 	$(INSTALL) $(ALL_PROGRAMS) '$(DESTDIR_SQ)$(gitexec_instdir_SQ)'
 	$(INSTALL) git$X git-upload-pack$X git-receive-pack$X git-upload-archive$X git-shell$X git-cvsserver '$(DESTDIR_SQ)$(bindir_SQ)'
 	$(MAKE) -C templates DESTDIR='$(DESTDIR_SQ)' install
+ifndef NO_PERL
 	$(MAKE) -C perl prefix='$(prefix_SQ)' DESTDIR='$(DESTDIR_SQ)' install
+endif
 ifndef NO_TCLTK
 	$(MAKE) -C gitk-git install
 	$(MAKE) -C git-gui gitexecdir='$(gitexec_instdir_SQ)' install