ruby: convert to autotools and fix build

ruby is converted to the autotools infrastructure, and the dependency
on host-ruby is added since a ruby interpreter is needed to build a
ruby interpreter. Fortunately, this is taken into account in the ruby
build process, and it first start to build a mini-interpreter that is
used to build the rest. However, this doesn't take the
cross-compilation case into account, so we have to build ruby for the
host.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>