Using core/sys/posix/mqueue.d on FreeBSD

[Arjan]

I'm using posix mqueue in a D application on Linux. Works fine.
But on FreeBSD it fails to compile due to the version statement:

[version (CRuntime_Glibc):](
https://github.com/dlang/dmd/blob/0cfdd7a589fd34fdf91db72e4999b4920efe5dc2/druntime/src/core/sys/posix/mqueue.d#L31)

While the file seems to be perfectly fine for FreeBSD too.

If I comment that line out, the app builds but have to supply the 
linker the lib -ltr in dub.sdl to get it linking too:

```
lflags "-lrt"
```

Which I don't have to do on Linux.

How to process from here to supply a PR?
Supposedly that version statement is there for a reason. How do I 
also incorporate FreeBSD?