ENH: Allow user code to block loading of CMake extensions by setting VTK_NO_LOAD_CMAKE_EXTENSIONS.

UseVTK.cmake.in

@@ -31,6 +31,8 @@ IF(NOT VTK_USE_FILE_INCLUDED)
   INCLUDE(${VTK_CMAKE_DIR}/vtkMakeInstantiator.cmake)
 
   # Load VTK's CMake extension commands.
-  VTK_LOAD_CMAKE_EXTENSIONS(${VTK_CMAKE_EXTENSIONS_DIR})
+  IF(NOT VTK_NO_LOAD_CMAKE_EXTENSIONS)
+    VTK_LOAD_CMAKE_EXTENSIONS(${VTK_CMAKE_EXTENSIONS_DIR})
+  ENDIF(NOT VTK_NO_LOAD_CMAKE_EXTENSIONS)
 
 ENDIF(NOT VTK_USE_FILE_INCLUDED)