message: Update `get_boost.sh` for caching
  Summary:
  Pull Request resolved:
  Download the `boost` sources into `boost_cache` to potentially reuse. Will help with CircleCI.
  Also refactor a bit to simplify version bumps in the future.
diff: # This source code is licensed under the MIT license found in the
  # LICENSE file in the root directory of this source tree.
  +BOOST_VERSION="1.71.0"
  +
  +BOOST_VERSION_UNDERSCORE="${BOOST_VERSION//./_}"
  +BOOST_FILE="boost_${BOOST_VERSION_UNDERSCORE}.tar.bz2"
  +BOOST_TAR_URL="https://boostorg.jfrog.io/artifactory/main/release/${BOOST_VERSION}/source/${BOOST_FILE}"
  +BOOST_CACHE_DIR="boost_cache"
  +BOOST_TAR_LOCAL="${BOOST_CACHE_DIR}/${BOOST_FILE}"
  +BOOST_DIR="boost_${BOOST_VERSION_UNDERSCORE}"
  +
  set -e
  -wget https://boostorg.jfrog.io/artifactory/main/release/1.71.0/source/boost_1_71_0.tar.bz2
  -tar --bzip2 -xf boost_1_71_0.tar.bz2
  -cd boost_1_71_0
  +# Check for cached artifacts.
  +if [ ! -d "$BOOST_CACHE_DIR" ] ; then
  + mkdir boost_cache
  +fi
  +if [ ! -f "$BOOST_TAR_LOCAL" ] ; then
  + wget "$BOOST_TAR_URL" -O "$BOOST_TAR_LOCAL"
  +fi
  +
  +tar --bzip2 -xf "$BOOST_TAR_LOCAL"
  +
  +cd "$BOOST_DIR"
  ./bootstrap.sh --with-libraries=filesystem,iostreams,program_options,regex,system,thread
  ./b2 -d0 install
  