git-gui: Honor a config.mak in git-gui's top level

I keep forgetting to include TCLTK_PATH when I build git-gui on some
systems.  Placing that rule (among others) into a config.mak makes it
easier to compile the application the same way every time.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

.gitignore

 .DS_Store
+config.mak
 Git Gui.app*
 git-gui.tcl
 GIT-VERSION-FILE

Makefile

@@ -99,6 +99,8 @@ ifeq ($(findstring $(MAKEFLAGS),s),s)
 QUIET_GEN =
 endif
 
+-include config.mak
+
 DESTDIR_SQ = $(subst ','\'',$(DESTDIR))
 gitexecdir_SQ = $(subst ','\'',$(gitexecdir))
 SHELL_PATH_SQ = $(subst ','\'',$(SHELL_PATH))