Support multiple module dependencies for the final prereq argument to the

APACHE_MODULE macro, as a comma separated list, e.g.

APACHE_MODULE(proxy_http2, [...], [proxy,http2])

will evaluate the availability of mod_proxy, followed by mod_http2


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1749658 13f79535-47bb-0310-9956-ffa450edef68