docs: def setup_build_path(build_path):
      """
      Create build directory. If this already exists, print informative
      error message and quit.
      """
      if os.path.isdir(build_path):
          fname = os.path.join(build_path, 'CMakeCache.txt')
          if os.path.exists(fname):
              sys.stderr.write('aborting setup\n')
              sys.stderr.write(
                  'build directory {0} which contains CMakeCache.txt already exists\n'.
                  format(build_path))
              sys.stderr.write(
                  'remove the build directory and then rerun setup\n')
              sys.exit(1)
      else:
          os.makedirs(build_path, 0o755)