Well, that didn't seem to fix the "make test" issue, but it did fuck me in other ways, since the list of header files created in the Makefile don't necessarily exist in older versions of Perl. As a result, I pretty much can't build anything that uses XS now.
I'm not mad at schwern (if you're reading this) - he's done *way* too much good work for me to be mad. Just frustrated is all. Perhaps there's an easy way to undo this but it's late and I'm tired. Seems miyagawa already submitted a ticket on this to rt.cpan. I added a comment.