So I'm running Debian Wheezy amd64 which has a feature called multiarch. Basically 32-bit libraries and 64-bit libraries are stored in different directories. For instance, on Debian amd64, libFLAC++.so isn't stored in /usr/lib or /usr/local/lib as you might expect, but in /usr/lib/x86_64-linux-gnu. Interestingly, cmake reports that it "-- Found FLAC: /usr/lib/libFLAC.so /usr/lib/libFLAC++.so" when neither of these files exist on my system. This causes make to fail when it tries to build one of the targets that requires libFLAC++.so. Now I could put a symlink at /usr/lib/libFLAC++.so to the libFLAC++ in the multiarch directory but that defeats the point of multiarch.
EDIT: After putting in a symlink at /usr/lib/libFLAC++.so, I found there's some other files that cmake doesn't find. These are: