Toggle on the build of fpcalc, a commandline tool to compute the
fingerprint of audio files.

It's less than 40kb and does not require additional dependencies.
The size of the closure (as returned by ~guix size chromaprint~) is
510.3 MB, just like before the patch.

Considering how light it is, I don't think it's worth making a separate
output.

--
Pierre Neidhardt