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
If you're feeling really lovely: can a test be added to do something more substantial than e.g. `--version` or `--help`? See `cmake.rb` for an example of an application formula with a good test and `tinyxml2.rb` for an example of a library formula with a good test. Thanks!