Autogen 5.18.2

Updated Autogen to 5.18.2 and inserts # explanation as to why the update isn't and shouldn't be to the newest version.

See [this PR](https://github.com/Homebrew/homebrew/pull/30406) for full information on the ongoing 5.18.3 'guile' issues.
autogen: add 5.18.2 bottle.