MinGW and why Linux users should care

(OS-9? You poor sod. Even HP-UX 6 was better than that, IIRC, although
that's not saying much.)

autoconf makes cross-compilation trivial? Sure, but, as you said, this is
only true *if* you don't use AC_TRY_RUN and friends, or otherwise need
accurate runtime tests, and a *lot* of programs fall into one category or
the other. If you do, you're screwed (or you have to do them *really* at
runtime rather than at configure time, which is not always practical).

It's certainly easier to cross-compile an autoconfiscated project than a
non-autoconfiscated project, but not necessarily all that much easier. As
ever, it really depends on whether the project has been written with an
eye to portability...

(Do people really have binary *-config scripts? Who? I mean, even
guile-config is a script. A Guile script, but still.)

I second the mention of gnulib: it's wonderful and maintained by a
cross-section of fine portability wizards, largely as an offshoot of
maintaining other GNU software... but it has no official releases or
(naturally) stable API, the churn rate is quite high, and if you're not
using autoconf, you might as well give up hope of using it without
significant pain. (Also, quite a lot of it is GPLv3ed, but the gnulib
maintainers are often amenable to requests to loosen the license to v2+ or
LGPL.)