### What it does
Checks for cfg attributes having operating systems used in target family position.

### Why is this bad?
The configuration option will not be recognised and the related item will not be included
by the conditional compilation engine.

### Example
```
#[cfg(linux)]
fn conditional() { }
```

Use instead:
```
#[cfg(target_os = "linux")]
fn conditional() { }

// or

#[cfg(unix)]
fn conditional() { }
```
Check the [Rust Reference](https://doc.rust-lang.org/reference/conditional-compilation.html#target_os) for more details.