FindProtobuf: add optional export declaration macro to generated cpp files
See https://groups.google.com/forum/#!topic/protobuf/PDR1bqRazts for more details.
Topic-rename: FindProtobuf-export-macro
See https://groups.google.com/forum/#!topic/protobuf/PDR1bqRazts for more details.
Topic-rename: FindProtobuf-export-macro