[chubot]

Hm what's an example of those invalid C constructs? I'd be interested in seeing what happened

One answer is the __has_feature tests mentioned in a sibling comment. Then you are using a supported API, not arbitrary code. Browsers should probably support something like that, if they don't already.

But the arbitrary code is still a useful fallback, for when the platform itself doesn't support config probing

I think you're saying that "writing good ./configure is hard", which is absolutely true. But it's still true that feature detection is better than version detection.