hhvm --HEAD depends OCaml

 * [x] remove boostfb, fix path `--HEAD`
 * [x] `--HEAD` depends OCaml
 * [x] `--HEAD` fix `HHVM_LINK_LIBRARIES` and `HHVM_EXTENSION` macros
 * [ ] Add extensions [`HHVM other extension share`](https://github.com/facebook/hhvm/wiki/HHVM-other-extension-share) [`Extensions`](https://github.com/facebook/hhvm/wiki/Extensions)