Rollup merge of rust-lang#46275 - dtolnay:compiletest-libc, r=Mark-Simulacrum

Compiletest libc dependency can be unix-only

In main.rs libc is imported as:

```rust
#[cfg(unix)]
extern crate libc;
```

This came up in Manishearth/compiletest-rs#90.

Author: kennytm

src/tools/compiletest/Cargo.toml

@@ -10,6 +10,8 @@ filetime = "0.1"
 getopts = "0.2"
 log = "0.3"
 rustc-serialize = "0.3"
+
+[target.'cfg(unix)'.dependencies]
 libc = "0.2"
 
 [target.'cfg(windows)'.dependencies]