Summary:
This PR changes the cmake of Caffe2 to look for system dependencies before resorting to the submodules in `third-party`. Only googletest should logically be in third-party, the other libraries should ideally be installed as system dependencies by the user. This PR adds system dependency checks for Gloo, CUB, pybind11, Eigen and benchmark, as these were missing from the cmake files.
In addition it removes the execution of `git submodule update --init` in cmake. This seems like bad behavior to me, it should be up to the user to download submodules and manage the git repository.
Closes https://github.com/caffe2/caffe2/pull/382
Differential Revision: D5124123
Pulled By: Yangqing
fbshipit-source-id: cc34dda58ffec447874a89d01058721c02a52476
