On the trunk:

mod_http2: disable and give warning when mpm_prefork is encountered. 
     The server will continue to work, but HTTP/2 will no longer be negotiated.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1800689 13f79535-47bb-0310-9956-ffa450edef68