def should_build_with_cython(package, release=None):
    """Returns the previously used Cython version (or 'unknown' if not
    previously built) if Cython should be used to build extension modules from
    pyx files.  If the ``release`` parameter is not specified an attempt is
    made to determine the release flag from `astropy.version`.
    """
    try:
        version_module = __import__(package + '.cython_version', fromlist=[
            'release', 'cython_version'])
    except ImportError:
        version_module = None
    if release is None and version_module is not None:
        try:
            release = version_module.release
        except AttributeError:
            pass
    try:
        cython_version = version_module.cython_version
    except AttributeError:
        cython_version = 'unknown'
    have_cython = False
    try:
        import Cython
        have_cython = True
    except ImportError:
        pass
    if have_cython and (not release or cython_version == 'unknown'):
        return cython_version
    else:
        return False
