message: Don't include pybind11 if its target is alreadt exported
diff: @@ -15,7 +15,11 @@ if (WIN32)
  set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} /WX")
  set(CMAKE_STATIC_LINKER_FLAGS "${CMAKE_STATIC_LINKER_FLAGS} /WX")
  endif()
  +
  +# ---[ pybind11
  +if (NOT TARGET pybind11)
  add_subdirectory(third_party/pybind11)
  +endif()
  # Customized version of find Protobuf. We need to avoid situations mentioned
  # in https://github.com/caffe2/caffe2/blob/b7d983f255ef5496474f1ea188edb5e0ac442761/cmake/ProtoBuf.cmake#L82-L92
  