How can I restrict a library to be only included under a certain OS?

[solidstate1991]

I added this line to my SDLang formatted dub build file:

```
dependency "libx11" version="0.0.1" platform="posix"
```

Yet it's included even on Windows, even if I change it to "linux".

Since there's a fatal bug in the library that disallows it to be 
built, I'll be leaving it out for now, and instead move onto 
something else, or try to issue a correction.