Created by: wangkuiyi

Currently, we use CMakeLists.txt to check if dependencies, like gflags, glog, and protobuf, have been installed. For example: https://github.com/PaddlePaddle/Paddle/blob/develop/cmake/FindGflags.cmake#L147.

An alternative is to use git add submodule to create dependencies in Paddle's repo pointing to https://github.com/gflags/gflags, https://github.com/google/glog, and https://github.com/google/protobuf.

There is no much difference between these two solutions as we build and run Paddle using Docker. A minor advantage of the latter one might be exposing dependencies of Paddle explicitly.