libc test builds two different versions of libc

```
cd /Development/source/libc/libc-test
cargo test
```

In output:
```
Compiling libc v0.2.142
...
Compiling libc v0.2.142 (/Development/source/rust/libc)
```

At least on Haiku, this causes a linking error as the current libc on crates.io hasn't been updated with a fix to a linking issue, which has been fixed in my local repo (and has landed here, a new release hasn't been cut yet).