Add CPack Support

There are proposals to replace script [`hhvm/packaging/blob/hhvm/deb/package`](https://github.com/hhvm/packaging/blob/master/hhvm/deb/package)
Most likely it will need to adapt the code, more dependencies (supplied separately).
Unfinished prototype CPack skeleton.

```
$ cmake -DCPACK_GENERATOR=ON .
$ make package
```

The HHVM team's not going to support CPack. Do you have an alternative contact or can this be omitted?