Every Rubyist is aware of Minero Aoki's ever useful setup.rb script. It's how most of us used to install our Ruby programs before RubyGems came along. And it's still mighty useful in certain scenarios, not the least of which is the job of the distribution package managers. While still providing the usual setup.rb script that one can distribute with a project, Setup also works as a stand-alone application. Instead of distributing setup.rb with a package, just instruct your users to install Ruby Setup and use it instead.
Gemfile gem 'setup', '~> 5.2.0'
install gem install setup