message: build: update protobuf to v3.4.0.
diff: set -e
  -# Unless overriden with an explicit release tag, e.g. v3.2.0rc2, we use a pinned
  -# HEAD commit. This is only until we get a release with
  -# https://github.com/google/protobuf/pull/3327, i.e. v3.4.0.
  -[ -z "$ENVOY_PROTOBUF_COMMIT" ] && ENVOY_PROTOBUF_COMMIT=062df3d0724d9ae5e3c65d481dc1d3aca811152e # 2017-07-20
  +VERSION=3.4.0
  -git clone https://github.com/google/protobuf.git
  -rsync -av protobuf/* $THIRDPARTY_SRC/protobuf
  -cd protobuf
  -git reset --hard "$ENVOY_PROTOBUF_COMMIT"
  -./autogen.sh
  +wget -O protobuf-$VERSION.tar.gz https://github.com/google/protobuf/releases/download/v$VERSION/protobuf-cpp-$VERSION.tar.gz
  +tar xf protobuf-$VERSION.tar.gz
  +rsync -av protobuf-$VERSION/* $THIRDPARTY_SRC/protobuf
  +cd protobuf-$VERSION
  ./configure --prefix=$THIRDPARTY_BUILD --enable-shared=no
  make V=1 install
  