Expose is_flag_supported

[kornelski]

Would it be possible to expose is_flag_supported in the public API?

I want to set -march={}, but the compiler may be older than the desired target architecture and reject it. I can't just use flag_if_supported, because if the flag is rejected, I don't want to ignore it. I want to set other flags as a fallback instead, and the fallback flags shouldn't be set unconditionally. (related to #268)

[alexcrichton]

Seems plausible to me!