git-gui: Allow gitexecdir, INSTALL to be set by the caller.

When used as a subproject within git.git our Makefile must honor
the gitexecdir which git.git's Makefile is passing down to us,
ensuring that we install our executables into the libexec chosen
by the end-user or packager.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Makefile

@@ -12,12 +12,16 @@ ifndef SHELL_PATH
 	SHELL_PATH = /bin/sh
 endif
 
+ifndef gitexecdir
 	gitexecdir := $(shell git --exec-path)
+endif
+
+ifndef INSTALL
 	INSTALL = install
+endif
 
 DESTDIR_SQ = $(subst ','\'',$(DESTDIR))
 gitexecdir_SQ = $(subst ','\'',$(gitexecdir))
-
 SHELL_PATH_SQ = $(subst ','\'',$(SHELL_PATH))
 
 $(patsubst %.sh,%,$(SCRIPT_SH)) : % : %.sh