Track external (pkgconfig) dependencies for custom setup script

The [`haskell-gi`-based packages](https://github.com/haskell-gi/haskell-gi) are heavily relying on a custom `Setup.hs` that generates the haskell bindings to `glib`-based C libraries. Consequently, this process depends on the C library at setup-time, but it seems cabal only makes it possible to express a build-time dependency (through `pkgconfig-depends` within the `library` section). In systems like NixOS, this distinction matters a lot.
Are you planning on supporting this use case in the future ? Otherwise, how would you suggest to overcome this untracked dependency issue ?

Thank you.