protoc: support '=' in --proto_path arguments
Sent upstream:
https://github.com/google/protobuf/pull/879
(cherry picked from commit 44c36312fe7eb2d77f8b5ea8bc5d2adc50bc5a41)
Sent upstream:
https://github.com/google/protobuf/pull/879
(cherry picked from commit 44c36312fe7eb2d77f8b5ea8bc5d2adc50bc5a41)