I wouldn't dream of "full portability". To me, that includes Linux, Windows, various BSDs, FreeDOS and places with 12-bit bytes. Even outside of practical considerations, to me "full portability" is a phrase that represents a paradox.
The way I see it, you have a tester program with a database of things to check against, and you give it an operational mode for the classes you care about. I.e. "warn me about anything you know of that's incompatible between Xorg and XFree86. Give me an error for anything that's incompatible between FreeBSD, OpenBSD and GNU." or "Give me an error for anything incompatible with unix-like systems [where that would mean common Linux environments, FreeBSD, OpenBSD, QNX...]"
I'm not suggesting the programmer should be demanded to provide support for all known platforms. I'm suggesting a programmer should be able to decide what platforms he'd like to support, get an advance rough idea at the parts of his program that are incompatible with that set, cull his supported platform set down to something manageable, and manage that.
The tool I'm describing would also be useful for people doing the active porting work, since it can immediately draw their attention to the places which need work.