Given an environment variable name and specified paths,
        return a pathsep-separated string of paths containing
        unique, extant, directories from those paths and from
        the environment variable. Raise an error if no paths
        are resolved.

def _build_paths(self, name, spec_path_lists, exists):
        """
        Given an environment variable name and specified paths,
        return a pathsep-separated string of paths containing
        unique, extant, directories from those paths and from
        the environment variable. Raise an error if no paths
        are resolved.
        """
        # flatten spec_path_lists
        spec_paths = itertools.chain.from_iterable(spec_path_lists)
        env_paths = safe_env.get(name, '').split(os.pathsep)
        paths = itertools.chain(spec_paths, env_paths)
        extant_paths = list(filter(os.path.isdir, paths)) if exists else paths
        if not extant_paths:
            msg = "%s environment variable is empty" % name.upper()
            raise distutils.errors.DistutilsPlatformError(msg)
        unique_paths = self._unique_everseen(extant_paths)
        return os.pathsep.join(unique_paths)