When building xmobar on NixOS (where stack's Nix integration is enabled by
default), this ensures that all required system libraries are available inside
the local build environment.
See https://docs.haskellstack.org/en/stable/nix_integration/ for details.
But, since simpleHTTP doesn't support HTTPS, the plugin only works
when one compiles with `with_counduit`, which brings in an insane
number of new deps.
See #378