make internal protobuf-c optional

[tuxmaster]

Most Linux distributions will still have protobuf-c, so the system one shout be uses instant the internal one. And the internal only if the system don't provide it.

[bowlofeggs]

I would like to package this library for Fedora, but the bundled protobuf-c does present an issue for me. Fedora already has protobuf-c included in the distribution, so it would be ideal to use that. However, in the absence of that I can include the bundled one, but I need to declare its version. What is the version of the bundled protobuf-c?

What would it take to get the library to link with a system-installed protobuf-c?