初次安装，运行gcc，按理说应该提示clang: error: no input files，结果提示这个。意思就是你没有同意协议，所以不能执行。要执行就要同意协议。
- 提示
$ gcc
Agreeing to the Xcode/iOS license requires admin privileges, please run “sudo xcodebuild -license” and then retry this command.- 执行
sudo xcodebuild -license按多次空格，直到出现下述文字后，输入agree回车：
By typing 'agree' you are agreeing to the terms of the software license agreements. Type 'print' to print them or anything else to cancel, [agree, print, cancel] 
