You are right, there are no .pc files in libtool. I investigated further, and I noticed the libgphoto2 configure script compiles a small C program to test for libltdl. It turns out the error was mine. I build my extensions in a chroot environment. I mistakenly omitted libltdl from the list of extensions to load before building. Once I found that and included it, libgphoto2 built properly.
Thanks!