Building rust with system LLVM

I must admit that I can't really get my head around rusts build system.

Basically I'd like to do packaging of 1.18 for Gentoo.

Now I guess in order to not use the LLVM which is in the rust repository I'd use the --llvm-root configure switch, which I did. However, rust still tries to compile its own LLVM for whatever reason.

Any ideas, why this is so?

I'm going to post some logs as soon as I'm at home.