Not that I'm defending autoconf (I dislike it a lot), but POSIX is also ignorant sometimes, so it shouldn't be treated dogmatically - e.g., google for the socklen_t confusion.

Other than that, the real world isn't black and white either. Although many unixen claim to be POSIX compliant, based on my experience they are mostly POSIX-ish.

And there's also no guarantee for sizeof(size_t) being a platform's word size.

> POSIX says that it's <sys/stat.h>. Problem solved.

Sadly what POSIX says is cold comfort when you're trying to compile something and the computer in front of you has put stat.h somewhere else.