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

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.

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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