Use system libs option

This adds the ability to use system libraries for hosts that already have `libzmq` and `libczmq` installed by passing the `--with-system-libs` option when installing:

```
gem install rbczmq -- --with-system-libs
```

I've been using it in production for a few months, and figured I'd pass it on in case you'd find it useful.

Thanks for a great library!