def read_manifest():
    """Read existing configuration from MANIFEST.in.

    We use that to ignore anything the MANIFEST.in ignores.
    """
    # XXX modifies global state, which is kind of evil
    if not os.path.isfile('MANIFEST.in'):
        return
    ignore, ignore_regexps = _get_ignore_from_manifest('MANIFEST.in')
    IGNORE.extend(ignore)
    IGNORE_REGEXPS.extend(ignore_regexps)