emerge libtheora doesn't install the theora.pc. pkg-config --list-all doesn't list theora. The theora.pc file is a part of libtheora. This can be verified by installing libtheora according to Theora's official installation instructions. This omission causes impossibility to compile a program using Theora from source if there is no Gentoo package available for this program.
Which version of libtheora misses it? I just rebuilt 1.0_alpha5 and it has the right theora.pc file.
1.0_alpha3 gentoo package misses the theora.pc file.