After much experimentation, I got Emacs to build and run on Tiger. I ran mac/make-package instead of the usual configure/make/install cycle, as advised by Stefan Tilkov. That built an installer instead of the usual Emacs.app, but the installer worked fine. (I started with a clean checkout, but make-package wouldn’t bootstrap so I just copied a bootstrapped lisp directory over from one of my failed attempts and that worked.)
So it seems that something is messed up in the main emacs configure script in CVS, since I saw someone else suggest commenting out the link to fink. I may have lost ispell, but at least I have Emacs back.