how portable is fchdir?

[Jim Meyering]

Do any of you know of reasonably modern systems
that do *not* support fchdir?

I know that POSIX requires it and that most systems provide it.
However, I've heard that on at least SunOS4 it fails for file systems
on which some type of accounting is enabled.
But SunOS4 is not exactly modern.

Also, it looks like SCO Xenix lacks the function altogether,
but that's hardly modern either.