>.."distribute on the App Store, without an explicit exception".
Otherwise, my goal with using GPL is to preserve the concept of copyleft with the library. The GPL only applies if you plan to distribute libzdb to third parties. In that case, my hope and the reason I use GPL is that any modification done to libzdb can be contributed back so others can benefit. Otherwise, you can do whatever you want with libzdb without any restrictions.
For instance, I can't find anything on the website that would explain how to submit a patch. There's a mailing list and it'd be great by 2002 standards. Hosting this on GitHub (or any other site with a forking workflow that's just as smooth) would probably solve your problem without having to resort to imposing legal restrictions. There still would be a few companies/people unwilling to help but only a considerable minority.