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.

What was the goal of this change? It doesn't look like `CFG_LIBDIR_RELATIVE` is put into the environment anywhere at build time here.