How to check -std=c89 or -std=gnu89 is set in C FE?

[Qing Zhao via Gcc-patches]

Hi, Joseph,

When -std=c89 or -std=gnu89 present in the command line, in C FE, which flags 
should be
checked to decide it’s -std=c89 or -std=gnu89?

Thanks a lot for your help.

Qing