Only override VTK_GROUP_ENABLE_Tk when DEFAULT

CMakeLists.txt

@@ -178,7 +178,11 @@ set("_vtk_module_reason_VTK::Java"
   "via `VTK_WRAP_JAVA`")
 # For vtkTkRenderWidget (only useful with Python, since WRAP_TCL is gone)
 if (VTK_USE_TK)
-  set(VTK_GROUP_ENABLE_Tk YES)
+  if (VTK_GROUP_ENABLE_Tk STREQUAL "DEFAULT")
+    set(VTK_GROUP_ENABLE_Tk "WANT")
+  elseif (VTK_GROUP_ENABLE_Tk STREQUAL "NO")
+    message(FATAL_ERROR "VTK_USE_TK=ON is not compatible with VTK_GROUP_ENABLE_Tk=NO")
+  endif ()
 endif ()
 if (VTK_USE_MPI)
   if (VTK_GROUP_ENABLE_MPI STREQUAL "DEFAULT")