Is this really true?  There is no reason you can't build against Gaim *without*
pkgconfig, really.

(Ultimately this is a Fedora policy question, of course, and as such doesn't
really affect upstream very much, I guess.)

Yes, it's a part of the package review guidelines:

http://fedoraproject.org/wiki/Packaging/ReviewGuidelines?action=diff&rev2=37&rev1=35

This will be in the next build.