Cross-platform availability of header files

[Zack Weinberg]

I've been doing research into the cross-platform availability of
header files that are commonly probed for in Autoconf scripts.
Results so far are here:
http://www.owlfolio.org/possibly-useful/notes-on-the-cross-platform-availability-of-header-files/
 Based on what I've learned, I have some questions for y'all:

1) What "interesting portability targets" have I left out?  I only
went back in time as far as FreeBSD 7, and I didn't even try to get my
hands on any of the surviving proprietary Unixes; is this too
shortsighted?

2) Autoconf currently probes for several of the headers in the "safely
assumed to exist everywhere" categories, notably in
AC_INCLUDES_DEFAULT.  It seems to me that this is unnecessary.  Would
patches to remove under-the-hood checks for the ubiquitous headers,
and deprecate macros that do explicit checks for them, be accepted?

3) It's a little tangential, but don't you think it's about time
AC_CHECK_HEADERS stopped doing all its tests two different ways?

Thanks,
zw