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 . -DINSTALL_LAYOUT=[DEB|RPM|MACPORTS]
$ make
$ cpack -G [DEB|RPM|MACPORTS]
```
[example cmake-cpack](http://www.cmake.org/Wiki/CMake:CPackPackageGenerators)

.gitignore, ref: facebook/hhvm#3060
@ptarjan yes!