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.

np. )

What I really would ask you to do -- test it on windows if you have no other complains. As I am really not sure whether I did not break something on this system.