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
Sorry, I am a bit late for the reply.
It takes almost 30s without checks, and 60s with these checks. 

I have update the rest part of the formula.If there is anything could be improved, please let me know.

Thanks,
Chilledheart