Fix --enable-modules=all breakage with mod_auth_db and mod_auth_digest

by allowing a module to disable itself if its prerequisites are not met.

This introduces the subtle nuance that if you request a module and you
don't meet its prerequisites, it'll refuse to build itself.

mod_ssl exits if its prerequisites are not met.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90261 13f79535-47bb-0310-9956-ffa450edef68