Update to work with libsnark:master

Snarkl was developed against a fork of libsnark at https://github.com/jkroll/libsnark (it's this libsnark that's built by ./prepare-depends.sh).  It should be ported to work with the master branch of https://github.com/scipr-lab/libsnark. Cf. #11 for a description of issues that may arise during this port.