fix for:
    _tkinter.TclError: Can't find a usable tk.tcl in the following directories: ...
based on https://github.com/NixOS/nixpkgs/commit/decd2feb0a1bc80940e697fa66e3b25383360c30
see also https://github.com/NixOS/nixpkgs/issues/1479
author: Kenneth Hoste (HPC-UGent)
--- tk8.6.4/unix/Makefile.in.orig	2015-10-29 18:57:12.213525347 +0100
+++ tk8.6.4/unix/Makefile.in	2015-10-29 19:06:19.397015702 +0100
@@ -1029,7 +1029,8 @@
 	$(CC) -c $(CC_SWITCHES) $(GENERIC_DIR)/tkVisual.c
 
 tkWindow.o: $(GENERIC_DIR)/tkWindow.c
-	$(CC) -c $(CC_SWITCHES) $(GENERIC_DIR)/tkWindow.c
+	$(CC) -c $(CC_SWITCHES) -DTK_LIBRARY=\"${TK_LIBRARY}\" \
+       $(GENERIC_DIR)/tkWindow.c
 
 tkButton.o: $(GENERIC_DIR)/tkButton.c
 	$(CC) -c $(CC_SWITCHES) $(GENERIC_DIR)/tkButton.c
--- tk8.6.4/generic/tkWindow.c.orig	2015-10-29 18:57:12.213525347 +0100
+++ tk8.6.4/generic/tkWindow.c	2015-10-29 19:03:30.156190540 +0100
@@ -988,6 +988,7 @@
 
     Tcl_SetVar2(interp, "tk_patchLevel", NULL, TK_PATCH_LEVEL, TCL_GLOBAL_ONLY);
     Tcl_SetVar2(interp, "tk_version",    NULL, TK_VERSION,     TCL_GLOBAL_ONLY);
+    Tcl_SetVar2(interp, "tk_library",    NULL, TK_LIBRARY,     TCL_GLOBAL_ONLY);
 
     tsdPtr->numMainWindows++;
     return tkwin;