Fix the interaction between --prefix and --with-layout. We were

over-writing the specified prefix during --with-layout processing.  By
keeping a copy of the specified prefix, and resetting it after the
layout processing, this is fixed.
PR:	7755


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90876 13f79535-47bb-0310-9956-ffa450edef68