Fix build failure with tcl 8.6

Index: opencascade/ros/src/Draw/Draw_Window.cxx
===================================================================
--- opencascade.orig/ros/src/Draw/Draw_Window.cxx
+++ opencascade/ros/src/Draw/Draw_Window.cxx
@@ -84,7 +84,7 @@
       errChannel = Tcl_GetStdChannel(TCL_STDERR);
       if (code != TCL_OK) {
         if (errChannel) {
-          Tcl_Write(errChannel, Interp->result, -1);
+          Tcl_Write(errChannel, Tcl_GetStringResult(Interp), -1);
           Tcl_Write(errChannel, "\n", 1);
         }
         Tcl_AddErrorInfo(Interp,
@@ -1055,7 +1055,7 @@
   mainWindow =
   Tk_MainWindow(interp) ;
   if (mainWindow == NULL) {
-    fprintf(stderr, "%s\n", interp->result);
+    fprintf(stderr, "%s\n", Tcl_GetStringResult(interp));
     exit(1);
   }
   Tk_Name(mainWindow) =
@@ -2060,10 +2060,10 @@
       OCC_CATCH_SIGNALS
       Standard_Integer res = Tk_Init (interp);
       if (res != TCL_OK)
       {
-        cout << "tkLoop: error in Tk initialization. Tcl reported: " << interp->result << endl;
+        cout << "tkLoop: error in Tk initialization. Tcl reported: " << Tcl_GetStringResult(interp) << endl;
       }
     }
     catch (Standard_Failure)
     {
       cout << "tkLoop: exception in TK_Init\n";
@@ -2071,8 +2071,8 @@
     Tcl_StaticPackage (interp, "Tk", Tk_Init, (Tcl_PackageInitProc* ) NULL);
     mainWindow = Tk_MainWindow (interp);
     if (mainWindow == NULL)
     {
-      fprintf (stderr, "%s\n", interp->result);
+      fprintf (stderr, "%s\n", Tcl_GetStringResult(interp));
       cout << "tkLoop: Tk_MainWindow() returned NULL. Exiting...\n";
       Tcl_Exit (0);
     }