code: def fetch_libcapnp(savedir, url=None):
      
      is_preconfigured = False
      if url is None:
          url = libcapnp_url
          is_preconfigured = True
      dest = pjoin(savedir, 'capnproto-c++')
      if os.path.exists(dest):
          info("already have %s" % dest)
          return
      fname = fetch_archive(savedir, url, libcapnp)
      tf = tarfile.open(fname)
      with_version = pjoin(savedir, tf.firstmember.path)
      tf.extractall(savedir)
      tf.close()
      # remove version suffix:
      if is_preconfigured:
          shutil.move(with_version, dest)
      else:
          cpp_dir = os.path.join(with_version, 'c++')
          conf = Popen(['autoreconf', '-i'], cwd=cpp_dir)
          returncode = conf.wait()
          if returncode != 0:
            raise RuntimeError('Autoreconf failed. Make sure autotools are installed on your system.')
          shutil.move(cpp_dir, dest)
docs: download and extract libcapnp