Building examples is attempted when imported as a library

Currently, `build.zig` always attempts to build all the examples, even when `zyph` is imported as a library. This is generally smelly and leads to needless build logs from the application using `zyph` as a dependency warning the user that the example directory failed to build. 

It would be much better if the examples binaries were only built when `zyph` itself is the root module.