Please pull my change to protobuf

Hi,

I made a change to the protobuf formula to compile and link cleanly with clang++ and libc++, this is related to this issue: http://code.google.com/p/protobuf/issues/detail?id=449

I made the necessary changes, please validate and pull as necessary.

Regards,
Ahmed

Signed-off-by: Ahmed Badran <ahmed.badran@gmail.com>