How to share methods between modules while maintaining best namespacing practices.

code_sharing.rb

module GemName
  module CoolClassInterface
    module Win32
      def func_a; end
      def func_b; end
    end
    module Linux
      def func_a; end # but now I don't have DRY code.  How do I make Win32 and Linux share this?
      def func_c; end
    end
  end
  class CoolClass
    include CoolClassInterface::Win32 if OS.windows?
    include CoolClassInterface::Linux if OS.linux?
  end
end

class_member = CoolClass.new # can definitely use func_a

solution_1.rb

module GemName
  module CoolClassInterface
    def func_a; end
  end
  module Win32
    # I get the functionality I want, but I lose namespacing.
    # I'd like to refer to CoolClassInterface::Win32.  Is that
    # even something I should want?
    include CoolClassInterface
    def func_b; end
  end
  module Linux
    include CoolClassInterface
    def func_c; end
  end
  class CoolClass
    include Win32 if OS.windows?
    include Linux if OS.linux?
  end
end

class_member = CoolClass.new # can definitely use func_a

solution_2.rb

module GemName
  module CoolClassInterface
    def func_a; end
    module Win32
      # I get the namespacing I want, but this seems bulky?
      # is including a module while nested inside that module
      # considered bad practice?
      include CoolClassInterface
      def func_b; end
    end
    module Linux
      include CoolClassInterface
      def func_c; end
    end
  end
  class CoolClass
    include CoolClassInterface::Win32 if OS.windows?
    include CoolClassInterface::Linux if OS.linux?
  end
end

class_member = CoolClass.new # can definitely use func_a