protobuf: resolve shared library multiple definition conflicts.

Patch to deal with protobuf problems where linking in multiple instances of protocol buffers via shared libraries causes crashes in Mac OS X.
Details  : https://bitbucket.org/osrf/release-tools/issue/25
Upstream : http://code.google.com/p/protobuf/issues/detail?id=128

patch courtesy of @j-rivero
https://github.com/j-rivero/homebrew-protobuf