cabal new-build (now the default with recent cabal versions) stores stuff
in dist-newstyle, versioned by the ghc version and the package version.

So, as propellor builds itself on a host over and over again,
and the host is upgraded and propellor is upgraded, dist-newstyle will
accumulate cruft used by old builds.

cabal clean can remove it of course, but then it won't update the build
incrementally.

What would be good is, a way to detect that the versioning has changed,
and only then run cabal clean. One way to do that, would be when updating
the propellor symlink to the cabal built binary, compare the old and new
binary location. If they're not the same, the versioning has changed,
and so cabal clean and re-build. --[[Joey]]

(Note that stack probably has the same problem too.)
