Handle lib paths with spaces in `compiler/rustc_llvm/build.rs`

[ChrisDenton]

In compiler/rustc_llvm/build.rs llvm-config is called to get a list of lib paths. By default this outputs paths separated by spaces, meaning that if the paths themselves contain spaces then the output will be messed up.

Fixing this requires using the --quote-paths option and handling quotes. However, we would then also need to handle \ escaping.

Alternatively we could just say this is a limitation when building llvm. But then we should at least document this somewhere.

[eggyal]

@rustbot claim