core: Build core functionality

With these changes we only build the Ruby interpreter. In the following patches
we will add Ruby extensions and encodings as well. The original Ruby build
provides what is called 'miniruby' - the minimalistic variant of the Ruby
intepretor and which doesn't have any extensions. We used the same idea to
provide a minimalistic version for Unikraft when the mini configuration is
checked via menuconfig.

Some of the sources need to be generated by the origin build system, that's why
we are running both `./configure` and `make` on the origin code before building
for Unikraft.

Signed-off-by: Andrei Gogonea <andrei.gogonea09@gmail.com>
Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Stefan Teodorescu <stefanl.teodorescu@gmail.com>

ruby/config.h: Disable features unavailable in Unikraft

Signed-off-by: Andrei Gogonea <andrei.gogonea09@gmail.com>
Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Stefan Teodorescu <stefanl.teodorescu@gmail.com>

ruby/config.h: Introduce generated configuration file

Signed-off-by: Andrei Gogonea <andrei.gogonea09@gmail.com>
Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Stefan Teodorescu <stefanl.teodorescu@gmail.com>

Introduce library skeleton

We use the BSD-3 license.

Signed-off-by: Andrei Gogonea <andrei.gogonea09@gmail.com>
Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Stefan Teodorescu <stefanl.teodorescu@gmail.com>

Initial commit (empty)

Signed-off-by: Andrei Gogonea <andrei.gogonea09@gmail.com>