Allow all parts of Apache 2.0 to build when --srcdir is used. This

Allow all parts of Apache 2.0 to build when --srcdir is used.  This
required exposing a build directory and a source directory to all parts
of Apache's build system.  It also required a small hack in APR-util, if
we are using the bundled Expat, and we are using VPATH support, then we
have hard-coded the xml/expat location.  I couldn't figure out how to
allow the configure script to determine the correct location.  I added a
comment, but if somebody else figures it out, we should fix that at some
point.

PR:	7630

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89517 13f79535-47bb-0310-9956-ffa450edef68