Provide (link to) instructions on how to install missing pkgconfig-depends

Obviously Cabal cannot and should not keep itself up to date with a massive database of pkg-config package names and distro packages....but Repology is a superb project which may be able to do just this. See https://github.com/repology/repology-updater/issues/1309 where this is proposed.

I suppose the simplest possible version, which spares Cabal from doing any possibly-sketchy IO, would be just to paste a link to a Repology query for the package(s) in question when it does the pkg-config check (at configure time) and find one (or more) missing. @AMDmi3 does this sound reasonable to you?

The info doesn't yet exist on Repology, but I am working on that (follow the links). I open this now in order to make sure this is as useful as I hope it should be. Cabal conveniently supports pkg-config in a "first class" way, and so is a good proving ground.