investigate alternate way for setting the version number

It was proposed to use the CPP extension instead of using a custom `Setup.hs` script that runs `git describe`. See discussion starting at https://github.com/avh4/elm-format/pull/559#issuecomment-426611296

Start by reading the custom Setup.hs script and the discussion linked from pull request 559, especially the proposal to use the CPP extension. Trace how the current script runs git describe to set the version, then determine whether the proposed approach covers the same behavior. Done means documenting or implementing a decided alternate version-setting mechanism.

Written by the indexing model from the issue text.