vtkRenderingOpenVR: Support building the module externally

This enables building the module against an existing VTK build tree so that
it can be packaged and distributed independently (e.g Slicer extension)

Rendering/OpenVR/CMakeLists.txt

-set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}")
+if(NOT VTK_SOURCE_DIR)
+  cmake_minimum_required(VERSION 3.3 FATAL_ERROR)
+  project(vtkRenderingOpenVR)
+  find_package(VTK REQUIRED)
+endif()
+set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${VTK_CMAKE_DIR}")
+if(NOT VTK_SOURCE_DIR)
+  include(vtkExternalModuleMacros)
+endif()
 
 find_package(OpenVR REQUIRED)