Use a fallback method for determining SOABI flags if the needed config
    var is unset or unavailable.

def get_flag(var, fallback, expected=True, warn=True):
    """Use a fallback method for determining SOABI flags if the needed config
    var is unset or unavailable."""
    val = get_config_var(var)
    if val is None:
        if warn:
            log.debug("Config variable '%s' is unset, Python ABI tag may "
                      "be incorrect", var)
        return fallback()
    return val == expected