Summary:
This basically allows Detectron to be built without having to include the many cmake files from Caffe2. Caffe2 now properly exports utility functions and dependent libraries, and so one can use the new style targets "caffe2_library" and "caffe2_gpu_library" directly.

This should also solve problems like #251 after we update the conda version. cc pjh5.
Closes https://github.com/facebookresearch/Detectron/pull/286

Reviewed By: ir413

Differential Revision: D7397696

Pulled By: Yangqing

fbshipit-source-id: 7a47e0addedd4b3c88dd8e2e130afdad04706acc