Fix path to the wish binary.

Index: tcl/pd-gui.tcl
--- tcl/pd-gui.tcl.orig
+++ tcl/pd-gui.tcl
@@ -1,6 +1,6 @@
 #!/bin/sh
 # This line continues for Tcl, but is a single line for 'sh' \
-    exec wish "$0" -- ${1+"$@"}
+    exec ${MODTK_BIN} "$0" -- ${1+"$@"}
 # For information on usage and redistribution, and for a DISCLAIMER OF ALL
 # WARRANTIES, see the file, "LICENSE.txt," in this distribution.
 # Copyright (c) 1997-2009 Miller Puckette.
