Look for crates in LIBDIR, provided by --libdir option

Fixies #11671

This commit changes default relative libdir 'lib' to a relative libdir calculated using LIBDIR provided by --libdir configuration option. In case if no option was provided behavior does not change.

See [this comment](https://github.com/rust-lang/rust/pull/16552#issuecomment-63916075) for some more info, but the gist is that our windows installation process wants dlls in the same dirs as the bins. In general I suspect that windows users rarely want this level of configuration anyway, so it's probably not too bad.