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

Since default Debian/Ubuntu mruby package does not enable C++ ABI
mode, it cannot be used with our C++ code, since mruby exception does
not properly cleanup C++ object allocated on stack.  We have to build
mruby with conf.enable_cxx_abi explicitly.  It is safe for us to build
our own mruby.