Make mpglib interface optional

This is a feature suggestion.

I believe mpglib is optional for some use cases.

I more or less did a parallel implementation of this repo on my local project before I found out this allyourcodebase org existed. In my local project, I was able to build and encode without including any of the C files under the mpglib directory.

I don't yet know how to add config options in a zig library, so that a dependent project can pass parameters and affect how a dependency's build.zig functions. But if that's possible, then adding a flag for excluding mpglib might be a good idea.

It could also be as simple as splitting it up into two build artifacts?