buildconf: add --help for the new behavior

buildconf's behavior is a little complicated (and was made more so with
apr-config mode), so add some self-documentation.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/trunk-buildconf-noapr@1780470 13f79535-47bb-0310-9956-ffa450edef68