extensions: Build extensions

extensions: Build extensions

All extensions are included in a new library, libruby_ext. Each extension has
its own configuration option and a <extconf.h> header. Given that the
extensions files are generated at build time by the origin build system, we
also have to run `./configure` and `make` for the original build before
building them for Unikraft.

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Stefan Teodorescu <stefanl.teodorescu@gmail.com>