commit 9575364d302eb5e6187724b122dc902041f38330
author Pieter Noordhuis <pietern@fb.com> Thu Oct 12 11:48:11 2017 -0700
committer Facebook Github Bot <facebook-github-bot@users.noreply.github.com> Thu Oct 12 11:48:50 2017 -0700
tree 254328308396edccc7941fc42d1aae07e84f648c
parent 1e8a16224f2a0fff7d6fef8809f9756f690ec4ab

Update protobuf detection

Summary:
The scripts/build_local.sh script would always build protoc from the
third_party protobuf tree and override the PROTOBUF_PROTOC_EXECUTABLE
CMake variable. This variable is used by the protobuf CMake files, so
it doesn't let us detect whether the protoc was specified by the user
or by the protobuf CMake files (e.g. an existing installation). This
in turn led to a problem where system installed headers would be
picked up while using protoc built from third_party. This only works
if the system installed version matches the version included in the
Caffe2 tree. Therefore, this commit changes the variable to specify a
custom protoc executable to CAFFE2_CUSTOM_PROTOC_EXECUTABLE, and
forces the use of the bundled libprotobuf when it is specified.

The result is that we now EITHER specify a custom protoc (as required
for cross-compilation where protoc must be compiled for the host and
libprotobuf for the target architecture) and use libprotobuf from the
Caffe2 tree, OR use system protobuf.

If system protobuf cannot be found, we fall back to building protoc
and libprotobuf in tree and packaging it as part of the Caffe2 build
artifacts.
Closes https://github.com/caffe2/caffe2/pull/1328

Differential Revision: D6032836

Pulled By: pietern

fbshipit-source-id: b75f8dd88412f02c947dc81ca43f7b2788da51e5