Tell cabal to configure zlib a certain way when building it, making its build
errors go away without changing our own cabal package configuration.

I almost used it to narrow these two dependencies to library-only but it turns
out to not make much difference so it doesn't seem worth the extra complexity
now.

However, having access to the library might prove useful in the future.

Each generated with `cabal2nix https://.../` and then mangled by alejandro