Improve pkg-config/dependencies installation story on Windows

Currently it's up to the Windows user (and undocumented!) to use `stack exec -- pacman` to set up `pkg-config` and any dependencies.

_At worst_, `stack` should be able to handle `pacman -Sy` and the `pkg-config` installation, and then provide a more helpful error message if a package is missing (i.e. how to search for and install it).

If a package's `pkg-config` name is always the suffix of its `pacman` package name -- I'm not sure of this, but it seems likely -- then I don't see why `stack` can't automate the whole process.