With some research from Pav Lucistnik <[EMAIL PROTECTED]>, a rogue file -- /usr/include/features.h -- was discovered and removed. It's dated Nov 21, 2001, and is claimed by no package so I have no idea how it got there. But as far as I can tell, that was the culprit.
I will add this info to the PR as well.
-- Paul Beard <www.paulbeard.org/> paulbeard [at] mac.com