mod_lua backport: Make httpd find lua 5.2 headers/lib during configure if present on the system (mod_lua is fully compatible with 5.2).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1605276 13f79535-47bb-0310-9956-ffa450edef68