Merge r1736250 from trunk:

mod_http2: no variadic macro in C89, use equivalent function.
Reviewed/backported by: ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1736251 13f79535-47bb-0310-9956-ffa450edef68