ruby: add support for gmp as an optional dependency

Ruby can now use gmp optionally, so make sure that if the gmp package
for the target is enabled, it gets built before the ruby package, and
the appropriate configure options are passed to Ruby.

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