Add support for non-standard WRKDIRPREFIX, which is commonly seen
on FreeBSD clusters with only one nfs mounted, read only ports tree.

The functionality of supporting non-standard WRKDIRPREFIX is just
supported in ordinary port, but not in port which crosses referenced
other port's WRKSRC during the build. This commit fixes it.

Reviewed by: statue@freebsd.sinica.edu.tw (MAINTAINER)