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.

Sorry, but I see in original issue (#11671), that rust was configured with --libdir. And installed with the same --libdir. Or am I wrong? Where is it written that --libdir has been changed between configure and install? May be I has not seen it, so if you can, please, point me to the place where it is stated.