Use --accept-proxy-protocol to enable PROXY protocol handling

This is because when we make tar ball, we are no longer under git
control.  git submodule does not work there.  We instead bundle mruby
source tree in archive.  git submodule command is only required when
building from git repository.

Refactor Http2Upstream so that we can share code between link header
field based push and mruby push.

Source files were downloaded from
https://bitbucket.org/birkenfeld/sphinx-contrib/src/default/rubydomain/

Setting unused -L really makes libtool get angry