Thanks to @chneukirchen for the suggestion.
This makes it easier to see what is supported by each OS, and to improve them individually.
Well, support may be too concrete of a term. There is a lot that is missing in OS X, and not everything can be implemented with portability shims. The time APIs in OS X seem to have frozen with NeXTSTEP and never advanced with POSIX.
First attempt at building a new OpenNTPD portable project, based on the latest OpenBSD source.