env do |env|
  env.add_builder(:Checker) do |params|
    unless @cache.up_to_date?(@target, :Checker, @sources, @env)
      puts "Checker #{@sources.first}" if @env.echo != :off
      @cache.register_build(@target, :Checker, @sources, @env)
    end
    true
  end
  env.Program("simple.exe", "simple.c")
  env.Checker(:checker, "simple.exe")
end
