Fix linking libvtkWrapping with Python wrapping and kits enabled on Mac

On Mac, the libvtkWrapping-8.1.1 library fails to link to libpython when
configuring with VTK_WRAP_PYTHON and VTK_ENABLE_KITS enabled.

This fixes the following error:

FAILED: lib/libvtkWrapping-8.1.1.dylib
Undefined symbols for architecture x86_64:
  "_PyCallable_Check", referenced from:
      vtkPythonAlgorithm::SetPythonObject(_object*) in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::ProcessRequest(vtkInformation*, vtkInformationVector**, vtkInformationVector*) in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::FillInputPortInformation(int, vtkInformation*) in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::FillOutputPortInformation(int, vtkInformation*) in vtkPythonAlgorithm.cxx.o
  "_PyErr_Clear", referenced from:
      GetPythonErrorString() in vtkPythonAlgorithm.cxx.o
  "_PyErr_Fetch", referenced from:
      GetPythonErrorString() in vtkPythonAlgorithm.cxx.o
  "_PyImport_ImportModule", referenced from:
      GetPythonErrorString() in vtkPythonAlgorithm.cxx.o
  "_PyInt_AsLong", referenced from:
      vtkPythonAlgorithm::CheckResult(char const*, vtkSmartPyObject const&) in vtkPythonAlgorithm.cxx.o
  "_PyInt_FromLong", referenced from:
      vtkPythonAlgorithm::FillInputPortInformation(int, vtkInformation*) in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::FillOutputPortInformation(int, vtkInformation*) in vtkPythonAlgorithm.cxx.o
  "_PyObject_Call", referenced from:
      GetPythonErrorString() in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::SetPythonObject(_object*) in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::ProcessRequest(vtkInformation*, vtkInformationVector**, vtkInformationVector*) in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::FillInputPortInformation(int, vtkInformation*) in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::FillOutputPortInformation(int, vtkInformation*) in vtkPythonAlgorithm.cxx.o
  "_PyObject_GetAttrString", referenced from:
      GetPythonErrorString() in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::SetPythonObject(_object*) in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::ProcessRequest(vtkInformation*, vtkInformationVector**, vtkInformationVector*) in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::FillInputPortInformation(int, vtkInformation*) in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::FillOutputPortInformation(int, vtkInformation*) in vtkPythonAlgorithm.cxx.o
  "_PyObject_Str", referenced from:
      vtkPythonAlgorithm::PrintSelf(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, vtkIndent) in vtkPythonAlgorithm.cxx.o
      GetPythonErrorString() in vtkPythonAlgorithm.cxx.o
  "_PySequence_Fast", referenced from:
      GetPythonErrorString() in vtkPythonAlgorithm.cxx.o
  "_PySequence_Size", referenced from:
      GetPythonErrorString() in vtkPythonAlgorithm.cxx.o
  "_PyString_AsString", referenced from:
      vtkPythonAlgorithm::PrintSelf(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, vtkIndent) in vtkPythonAlgorithm.cxx.o
      GetPythonErrorString() in vtkPythonAlgorithm.cxx.o
  "_PyTuple_New", referenced from:
      GetPythonErrorString() in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::SetPythonObject(_object*) in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::ProcessRequest(vtkInformation*, vtkInformationVector**, vtkInformationVector*) in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::FillInputPortInformation(int, vtkInformation*) in vtkPythonAlgorithm.cxx.o
      vtkPythonAlgorithm::FillOutputPortInformation(int, vtkInformation*) in vtkPythonAlgorithm.cxx.o

CMake/vtkModuleTop.cmake

@@ -353,13 +353,19 @@ macro(_vtk_build_module _module)
   add_subdirectory("${${_module}_SOURCE_DIR}" "${${_module}_BINARY_DIR}")
 endmacro()
 
+include(vtkTargetLinkLibrariesWithDynamicLookup)
+
 # Build all modules.
 foreach(kit IN LISTS vtk_modules_and_kits)
   if(_${kit}_is_kit)
     set(_vtk_build_as_kit ${kit})
     set(kit_srcs)
+    set(_optional_python_link)
     foreach(kit_module IN LISTS _${kit}_modules)
       list(APPEND kit_srcs $<TARGET_OBJECTS:${kit_module}Objects>)
+      if(${kit_module}_OPTIONAL_PYTHON_LINK)
+        set(_optional_python_link 1)
+      endif()
     endforeach()
 
     configure_file("${_VTKModuleMacros_DIR}/vtkKit.cxx.in"
@@ -398,6 +404,10 @@ foreach(kit IN LISTS vtk_modules_and_kits)
     target_link_libraries(${kit}
       LINK_PRIVATE ${kit_priv}
       LINK_PUBLIC  ${kit_pub})
+    if(_optional_python_link)
+      vtk_module_load(vtkPython)
+      vtk_target_link_libraries_with_dynamic_lookup(${kit} LINK_PUBLIC ${vtkPython_LIBRARIES})
+    endif()
     vtk_target(${kit})
   else()
     if(VTK_ENABLE_KITS)