Well, we usually do that on the grounds that the library isn't useful otherwise. From a completely uneducated standpoint, I can only assume that's the case here. If not, I suppose you'd want to model it after qcap and v4l2.
Eh, sorry, I forgot that libv4l2 is a bit of a special case, and we actually do need to load it dynamically.
A better model here is, I think, gphoto2. That is, unconditionally link against the library, but introduce a HAVE_VOSK definition that you'd guard out its actual usage with.