definable.rb

require 'active_support'

module Definable
  def definable(name)
    define_method name do |value|
      (const_get(:ClassMethods, false) rescue const_set(:ClassMethods, Module.new))
        .send(:define_method, name) { value }

      send(:define_method, name) { self.class.send name }
    end
  end
end

module A1
  extend Definable

  definable :nyan
end

module B1
  extend A1
  extend ActiveSupport::Concern

  nyan :nyan1
end

module A2
  extend Definable

  definable :nyan
end

module B2
  extend A2
  extend ActiveSupport::Concern

  nyan :nyan2
end

class C1
  include B1
end

class C2
  include B2
end

p C1.nyan #=> :nyan1
p C2.nyan #=> :nyan2