Use an installed Expat when available, rather than the builtin one.

Presuming the installed one is a shared library, then this should resolve
some of the longstanding issues with mod_perl / XML::Parser::Expat.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@91252 13f79535-47bb-0310-9956-ffa450edef68