Direct access to Tcl_Interp->result was deprecated in 8.6

--- a/modules/tk/tk.c
+++ b/modules/tk/tk.c
@@ -76,6 +76,7 @@ char *strchr (), *strrchr ();
 #endif
 
 #define TCL_THREADS
+#define USE_INTERP_RESULT
 #include <tcl.h>
 #include <tk.h>