git-gui: Makefile - Handle $DESTDIR on Cygwin

gg_libdir is converted to an absolute Windows path on Cygwin,
but a later step attempts to prefix $DESTDIR to install to a
staging directory. Explicitly separate the uses of gg_libdir for
these two purposes so installation to $DESTDIR will work.
Signed-off-by: NMark Levedahl <mdl123@verizon.net>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

Makefile

@@ -110,7 +110,7 @@ TCLTK_PATH_SED = $(subst ','\'',$(subst \,\\,$(TCLTK_PATH)))
 
 gg_libdir ?= $(sharedir)/git-gui/lib
 libdir_SQ  = $(subst ','\'',$(gg_libdir))
-libdir_SED = $(subst ','\'',$(subst \,\\,$(gg_libdir)))
+libdir_SED = $(subst ','\'',$(subst \,\\,$(gg_libdir_sed_in)))
 exedir     = $(dir $(gitexecdir))share/git-gui/lib
 
 GITGUI_SCRIPT   := $$0
@@ -119,11 +119,12 @@ GITGUI_MACOSXAPP :=
 
 ifeq ($(uname_O),Cygwin)
 	GITGUI_SCRIPT := `cygpath --windows --absolute "$(GITGUI_SCRIPT)"`
-	gg_libdir := $(shell cygpath --windows --absolute "$(gg_libdir)")
+	gg_libdir_sed_in := $(shell cygpath --windows --absolute "$(gg_libdir)")
 else
 	ifeq ($(exedir),$(gg_libdir))
 		GITGUI_RELATIVE := 1
 	endif
+	gg_libdir_sed_in := $(gg_libdir)
 endif
 ifeq ($(uname_S),Darwin)
 	ifeq ($(shell test -d $(TKFRAMEWORK) && echo y),y)