Fix deprecated use of cargo build directive

- new build.rs just calls make (importing header files did not seem
  straight-forward)

Fix for part of #14 

It would be nice to get rid of `config.c`, but I suppose if header files could already be easily included in Rust, it wouldn't need to be there in the first place.
This should be calling `.status()` instead of `.spawn()`, the former waits for the process to finish but the latter doesn't.

We can also go further and move the generated `config.rs` into the build output directory, since `include!(concat!(env!("OUT_DIR"), "/config.rs"))` should actually work now (that didn't work when I first ported this to Cargo).