subprocess_env vars HTTP2 and H2PUSH supported, export of mod_http2.h with...

subprocess_env vars HTTP2 and H2PUSH supported, export of mod_http2.h with OPTIONALs for others to directly look these up, similar to mod_ssl

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1721150 13f79535-47bb-0310-9956-ffa450edef68