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.

With the current implementation this will not work at all. It means even if user uses the same libdir during configure and install, rust will not work with it different from 'lib'. So this feature is already broken.

And I think it is not sane to change such things during install, not during configure, so we probably should somehow fix it, so user can not change libdir during installation.