message: ci: update to protobuf-3.2.0 and retain source in /thirdparty.
  We need 3.2.0 to support the Bazel build and also need to keep around the source tree to import the
  root protobuf.bzl from bazel/envoy_build_system.bzl.
diff: @@ -84,13 +84,13 @@ cd ..
  rm -fr cares* c-ares*
  # protobuf
  -wget https://github.com/google/protobuf/releases/download/v3.0.0/protobuf-cpp-3.0.0.tar.gz
  -tar xf protobuf-cpp-3.0.0.tar.gz
  -cd protobuf-3.0.0
  +wget https://github.com/google/protobuf/releases/download/v3.2.0/protobuf-cpp-3.2.0.tar.gz
  +tar xf protobuf-cpp-3.2.0.tar.gz
  +cd protobuf-3.2.0
  ./configure --prefix=$THIRDPARTY_BUILD --enable-shared=no
  make install
  +make distclean
  cd ..
  -rm -fr protobuf*
  # cotire
  wget https://github.com/sakra/cotire/archive/cotire-1.7.8.tar.gz
  