Give an error if hhvm.server.user is set but we're built without support for changing users

### HHVM Version

3.27.0

### Operating System and Version

MacOS High Sierra

### Standalone code, or other way to reproduce the problem

```
hhvm -m daemon -p 8080 -d hhvm.server.user=nobody
```

### Actual result

```
Log file not specified under daemon mode.\n\n
```

Server running in daemon mode under my current user.

### Expected result

An error message indicating that setting `hhvm.server.user` does nothing as HHVM was built with SKIP_USER_CHANGE set by cmake (always true on MacOS)