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.
diff --git a/Modules/_tkinter.c b/Modules/_tkinter.c
index 04150d1..1307053 100644
--- a/Modules/_tkinter.c
+++ b/Modules/_tkinter.c
@@ -1265,7 +1265,7 @@
 	       !errorInCmd)
 	{
 		int result;
-#ifdef THIS_CODE_IS_BUGGY
+#ifdef HAVE_PYTCL_WAITUNTILEVENT
 		result = Tcl_DoOneEvent(TCL_DONT_WAIT);
 		if (PyErr_CheckSignals() != 0)
 			return NULL;
@@ -1275,7 +1275,7 @@
 		   thread-safe, but it seems *rather* safe as long as
 		   no two threads call mainloop() simultaneously. */
 		Py_BEGIN_ALLOW_THREADS
-		result = Tcl_WaitForEvent((Tcl_Time *)NULL);
+		result = PyTcl_WaitUntilEvent();
 		Py_END_ALLOW_THREADS
 #else
 		result = Tcl_DoOneEvent(0);