class_methods.rb

class Stuff
  def self.conf(*names)
    names.each do |name|
      define_singleton_method(name) do
        @confs ||= {}
        @confs[name]
      end
      define_singleton_method("#{name}=") do |value|
        @confs ||= {}
        @confs[name] = value
      end
    end
  end

  conf :aws_access_key, :aws_secret_key
end

with_module.rb

module WithConf
  module ClassMethods
    def conf(*names)
      names.each do |name|
        define_singleton_method(name) do
          self.confs[name]
        end
        define_singleton_method("#{name}=") do |value|
          self.confs[name] = value
        end
      end
    end

    def confs
      @confs ||= {}
    end
  end

  def self.included(klass)
    klass.extend(ClassMethods)
  end
end

class Stuff
  include WithConf

  conf :aws_access_key, :aws_secret_key
end