Rewrite Tcl/Tk interpreter path for the GUI tools.

Rewrite Tcl/Tk interpreter path for the GUI tools.

--with-tcltk=/path/to/wish sets the TCLTK_PATH variable that is
used to substitute the location of the wish interpreter in the
Tcl/Tk programs.

New tracking file, GIT-GUI-VARS, was introduced: it tracks the
location of the Tcl/Tk interpreter and activates the GUI tools
rebuild if the interpreter path was changed. The separate tracker
is better than the GIT-CFLAGS: there is no need to rebuild the whole
git if the interpreter path was changed.

Signed-off-by: Eygene Ryabinkin <rea-git@codelabs.ru>