unavailable function 'system': not available on iOS

升级到xcode9后，会出现这个编译错误

解决方法：

1、用nftw替换system

http://www.cocoachina.com/bbs/read.php?tid=1725915&page=1&toread=1#tpc

https://stackoverflow.com/questions/45420717/call-to-unavailable-function-system-not-available-on-ios

2、用peopen替换system

http://blog.csdn.net/fzhlee/article/details/73296612

3、依据SDK版本关闭或者打开代码编译

#if __IPHONE_OS_VERSION_MAX_ALLOWED < 110000//__IPHONE_11_0sysetm("xxxx")#endif