Allow us to override the default --libdir from the config.layout

if it was specified to configure.

Submitted by:	Thom May <thom@planetarytramp.net>
Reviewed by:	Aaron Bannert


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94511 13f79535-47bb-0310-9956-ffa450edef68