git-gui: Make git-gui lib dir configurable at runime

Introduce the GIT_GUI_LIB_DIR environment variable, to tell git-gui
where to look for TCL libs.  This allows a git-gui which has been
built with a prefix of /foo to be run out of directory /bar.  This is
the equivalent of GIT_EXEC_PATH or GITPERLLIB but for git-gui's TCL
libraries.

Signed-off-by: David Turner <dturner@twitter.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

macosx/AppMain.tcl

@@ -1,5 +1,10 @@
 set gitexecdir {@@gitexecdir@@}
-set gitguilib  {@@GITGUI_LIBDIR@@}
+if { [info exists ::env(GIT_GUI_LIB_DIR) ] } {
+	set gitguilib $::env(GIT_GUI_LIB_DIR)
+} else {
+	set gitguilib {@@GITGUI_LIBDIR@@}
+}
+
 set env(PATH) "$gitexecdir:$env(PATH)"
 
 if {[string first -psn [lindex $argv 0]] == 0} {