Set `CMAKE_JS_NODELIB_DEF` and `CMAKE_JS_NODELIB_TARGET`

[kraenhansen]

Users of cmake-js are instructed to add the following to their CMakeLists.txt

if(MSVC AND CMAKE_JS_NODELIB_DEF AND CMAKE_JS_NODELIB_TARGET)
  # Generate node.lib
  execute_process(COMMAND ${CMAKE_AR} /def:${CMAKE_JS_NODELIB_DEF} /out:${CMAKE_JS_NODELIB_TARGET} ${CMAKE_STATIC_LINKER_FLAGS})
endif()

Which will generate a node.lib file exposed to the CMake project via CMAKE_JS_LIB.