env1 = Rscons::Environment.new(echo: :command) do |env|
  env['CFLAGS'] = '-O2'
  env.add_build_hook do |build_op|
    build_op[:vars]['CPPFLAGS'] = '-DSTRING="Hello"'
  end
  env.add_post_build_hook do |build_op|
    $stdout.puts "post #{build_op[:target]}"
  end
  env.Program('program.exe', Dir['src/*.c'])
end

env2 = env1.clone do |env|
  env.Program('program2.exe', Dir['src/*.c'])
end