Add an optional hack for threads in Tkinter.

Add an optional hack for threads in Tkinter.
This one works!  However it requires using a modified version of
tclNotify.c (provided), which requires access to the Tcl source
to compile it.  In order to enable this hack, add the following
to the Setup line for _tkinter:
   tclNotify.c -DHAVE_PYTCL_WAITUNTILEVENT -I$(TCL)/generic
where TCL points to the source tree of Tcl 8.0.  Other versions
of Tcl are not supported.

The tclNotify.c file is copyrighted by Sun Microsystems; the
licensing terms are in the file license.terms.  According to this
file, no further permission to distribute this is required,
provided the file license.terms is included.  Hence, I am checking
that in, too.