If you're using #xmonad on #freebsd, are trying to update to 0.18.1, have gcc installed, and are getting errors in `cabal build` about an unsupported flag for `ar`, then the the problem may be that gcc's `ar` is being used instead of llvm's `ar`. It can be worked around by adding `--with-ar=/usr/bin/ar` to the `cabal build` call. 🎉