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>