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 . -DBUILD_PACKAGING=ON
$ cpack
```

.gitignore, ref: facebook/hhvm#3060
@ptarjan `cpack -G DEB -R 3.2.0`