Most importantly, these are consumed in the tracker-miners.git project
so that it can run when built with tracker core as a Meson subproject.
This should theoretically allow building tracker-miners on systems that
don't have tracker installed at all, although I think some problems
remain with doing that.

Note that meson.source_root() must never be used in variables that
are consumed by a subproject, because in that situation they expand to
the root of the *toplevel project* and not the current subproject.