That is totally fair: even something as simple as a #define available inside of a header file could have allowed this to be discovered from the source code, which would have not only not broken any existing code, but also would have allowed existing implementations to not have unreasonable burdons.
I just have a difficult time with the rather strong terminology that the definition is broken: the definition is consistent, reasonable, and even somewhat workable... it is just not terribly useful, satisfying, or comforting; if anything, I'd say "incomplete".
(Although, given that it chose "unspecified" rather than "implementation-defined", one has to wonder whether some existing system actually did something horrific in this case and left non-deterministic behavior. I guess there could always be a constant for "you're screwed", though. ;P)
Sure, there's a way to tell. There are lots of ways of telling which platform you're on (#defines), and platforms can document the expected behavior. If a platform doesn't, then complain to the vendor.
Yes, it sucks to resort to conditional compilation around "close", but it's already been explained why it's necessary. POSIX was not created in a vacuum as a theoretically ideal OS specification.
For HP-UX, it's "close it again".
For AIX, it's apparently "everything is fine, don't close it again". Linux, too.
That could be OK, even when undocumented, IF we had a way to discover what had occurred. But we don't. close() isn't broken because it can behave in multiple ways that can't be predicted in advance, it's broken because there's no safe way to proceed.
And by the way, we do know something about the target when we're writing POSIX-compliant code. We know the target is POSIX-compliant. That's the whole point of POSIX.