protobuf: apply several improvements and add an option to do build-time checks

- add an option to do build-time checks
  and double the build time approximately if enabled

- protobuf is moved to github,
  see more https://groups.google.com/d/msg/protobuf/3qvNnYo8-SM/GSMYGGBNXfoJ

- provide better format
`system "make", "check" if build.with? "check"`