There is also a sense that it might be useful for Haskell implementations to provide a gtk-config type tool for letting a build system know where it wants to keep stuff...
Actually I think this is not the right way to go. It should be the package manager on the host system that tells the build system where it wants to keep stuff. Certainly the library needs to inform the Haskell implementation where it lives: that's what gtk-config does in essence, and what the GHC package system does in a more general way. But I don't see a need for the Haskell implementation to tell the library where it should place its files; indeed this is likely to conflict with OS package managers that like to keep control of where files are installed. Cheers, Simon