add alien_check_built_version method

This is to address #83, which hasn't gotten any commentary yet.

It documents the existing method `alien_check_installed_version` and creates and documents a second `alien_check_built_version`.

You can see how this can simplify an AB::MB subclass here:

https://github.com/plicease/Alien-gmake/commit/f303f30141112aa751b4e8dd5b3f3e31104f4004