def apple_deployment_target_flag(os_, os_version):
    """compiler flag name which controls deployment target"""
    flag = {'Macos': '-mmacosx-version-min', 'iOS': '-mios-version-min',
        'watchOS': '-mwatchos-version-min', 'tvOS': '-mappletvos-version-min'
        }.get(str(os_))
    if not flag:
        return ''
    return '%s=%s' % (flag, os_version)
