protocolbuffers/protobuf ? reverse-engineered prompt

Build me a clean, working version of Protocol Buffers from this repo, so I can compile .proto files and use the generated code in a simple example.

I want the main compiler and runtime to build successfully, and I want a small demo that shows a message being defined, generated, serialized, and read back again. If there are any missing setup steps or current build issues, please fix them and make the project easy to run from source. Use the existing docs and examples as a guide, and look up current build instructions online if you need to. Also make sure the basic install and usage flow is clear enough that someone new to protobuf could follow it without getting stuck.