def build_subprotocols(subprotocols):
    """
    Builds websocket subprotocol headers from the given subprotocol values.
    
    Parameters
    ----------
    subprotocols : `list` of `str`
        A list of supported subprotocols.
    
    Returns
    -------
    header_value : `str`
    """
    return ', '.join(subprotocols)