Hi, Protobuf 3.0.0-9.1 (the version in unstable) already depends upon libgtest-dev; the FTBFS is due to a missing build-dependency on libgmock-dev.
The version of protobuf in experimental (3.5.2-2) includes a build-dep on both libgtest-dev and libgmock-dev, so it should build properly with the latest googletest.
