Fix the EXPAT logic to the new meaning... Before, it was simply there to

check that expat-lite existed, which was kind of bogus... Now, we
allow it to choose which Expat we want. We prefer the system's Expat
if available but will use expat-lite as a backup. We can also bypass
that as well.

PR:
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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@91350 13f79535-47bb-0310-9956-ffa450edef68