Make sure ACL API support in mod_dav is not dependent on the presence

or absence of apr-util v1.6. Log a message to note that ACL support
is disabled with apr-util <= 1.5.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1824590 13f79535-47bb-0310-9956-ffa450edef68