fix build on OpenBSD

Hi, 
i tried YouCompleteMe on OpenBSD/amd64,-current and with these changes it works for me pretty well.
- expose a new flag (--system-boost) to
  build ycmd with system boost.
- don't link against librt on OpenBSD
- add "-pthread" to CXX flags on OpenBSD

boost from third_party didn't compile but building it against system boost did work for me.



i build YouCompleteMe with

```
./install.sh --clang-completer --omnisharp-completer --system-libclang --system-boost
```

Any Comments or Feedback?

Thanks for this great software,
Fabian