From the Heroku Python buildback, https://github.com/heroku/heroku-buildpack-python/blob/533def6b57a09a60c9dd7755958f12e56d3bf761/bin/steps/pylibmc#L19

pylibmc.bash

# If pylibmc exists within requirements, use vendored libmemcached.
if (grep -Eiq "\s*pylibmc" requirements.txt) then
  echo "-----> Noticed pylibmc. Bootstrapping libmemcached."
  cd .heroku

  if [ -d "vendor/lib/sasl2" ]; then
    export LIBMEMCACHED=$(pwd)/vendor
  else
    # Download and extract libmemcached into target vendor directory.
    curl -s -L -o tmp-libmemcached.tar.gz $VENDORED_MEMCACHED
    tar -zxvf tmp-libmemcached.tar.gz > /dev/null
    rm tmp-libmemcached.tar.gz
    export LIBMEMCACHED=$(pwd)/vendor
  fi

  export LIBMEMCACHED=$(pwd)/vendor
  cd ..
fi