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'd rather not ignore options based on the detected OSTYPE. What's keeping us from calculating the relative directory for windows builds the same way it's calculated for other builds, and just supply a different default CFG_LIBDIR_RELATIVE?