Add define controlled config.h include

This enables cleaner builds for those choosing to dynamically create `config.h`. I've got an initial implementation working in the CMake prototype.

I set the location at the top of `include` as a peer to `mruby.h` because I don't believe Ruby's arch specific location is desirable with mruby's expected use cases. That said, it's an easy change in CMake to match whatever location you want.