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.

I do not understand how can it be installed with different libdir, as you just unpack nightly and that's all, you do not call any install for it, or am I wrong?

Anyway in current implementation (without my changes) if you change libdir from default lib it will fail as well, independing on what you've passed to configure. How can it work in current version if somebody will change libdir on installation? It will fail.

Or may be I do not understand what you are saying. I'll describe here how I understand nightly binary installation:

nightly is built (with default libdir, yes), the somebody downloads and unpacks it to the folder he likes. No additional calling of install.sh or whatever happens, just copying.