build: Allow backend libraries to have individual API versions

The API version is included in the GIR file name, and only changes when
API is broken or removed. If new backends, such as the dummy backend,
are to have unstable APIs, having individual versions for each backend
library is necessary to allow backend API breaks without breaking the
whole of the folks API.

The API version has not been added to the VAPI file name, though it
should, because that would be a backwards-incompatible change. It can be
added when folks next breaks API compatibility.

https://bugzilla.gnome.org/show_bug.cgi?id=711544