message: ci: Move to GCC 5.4 (Xenial default) for prebuilts
  This is part 1 of a change to switch us over to a GCC 5.4 so that
  we get a thread safe std::string implementation. Will follow up
  with another commit with CI and doc changes.
diff: @@ -5,13 +5,7 @@ set -e
  # Setup basic requirements and install them.
  apt-get update
  apt-get install -y wget software-properties-common make cmake git python python-pip \
  - clang-format-3.6 bc libtool automake zip time
  -apt-get install -y golang
  -# For debugging.
  -apt-get install -y gdb strace
  -add-apt-repository -y ppa:ubuntu-toolchain-r/test
  -apt-get update
  -apt-get install -y g++-4.9
  + clang-format-3.6 bc libtool automake zip time golang g++ gdb strace
  # clang head (currently 5.0)
  wget -O - http://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -
  apt-add-repository "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial main"
  @@ -32,9 +26,14 @@ pip install virtualenv
  export GOPATH=/usr/lib/go
  go get github.com/bazelbuild/buildifier/buildifier
  -# GCC 4.9 for everything.
  -export CC=gcc-4.9
  -export CXX=g++-4.9
  +# GCC for everything.
  +export CC=gcc
  +export CXX=g++
  +CXX_VERSION="$(${CXX} --version | grep ^g++)"
  +if [[ "${CXX_VERSION}" != "g++ (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609" ]]; then
  + echo "Unexpected compiler version: ${CXX_VERSION}"
  + exit 1
  +fi
  export THIRDPARTY_DEPS=/tmp
  export THIRDPARTY_SRC=/thirdparty
  