Allow building without any git installed

This is a follow-up patch to 49fa65a7 (Allow the built-in exec path to be
relative to the command invocation path, 2008-07-23).  Without specific
gitexecdir passed from the command line, git-gui's build procedure would
try to figure out the value for it by running an installed git.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -1067,7 +1067,7 @@ endif
 
 all::
 ifndef NO_TCLTK
-	$(QUIET_SUBDIR0)git-gui $(QUIET_SUBDIR1) all
+	$(QUIET_SUBDIR0)git-gui $(QUIET_SUBDIR1) gitexecdir='$(gitexec_instdir_SQ)' all
 	$(QUIET_SUBDIR0)gitk-git $(QUIET_SUBDIR1) all
 endif
 	$(QUIET_SUBDIR0)perl $(QUIET_SUBDIR1) PERL_PATH='$(PERL_PATH_SQ)' prefix='$(prefix_SQ)' all