h2o: 2.2.6 -> 2.3.0-beta2, enable multi-output

This also lets us remove a hack for supporting LibreSSL 2.7, since we're
now using 2.9 by default, anyway.

Finally, use Ninja to run the CMake build instead of Make -- speeds
things up for me by a few seconds.

Signed-off-by: Austin Seipp <aseipp@pobox.com>