Update Autotools build for nettle library (fixes #5)

* Per the nettle source, openssl is optionally linked against
  for performance testing only; this causes problems on RedHat/CentOS,
  so is disabled here
* On 64-bit CentOS (and possibly other systems), default library
  path for autotools is `usr/lib64`; `BinDeps` only looks in `usr/lib`,
  so here, the default location is overridden to point to `usr/lib`.
  This is probably better handled in `BinDeps`, but fixing here for
  now so `Nettle` can be used.
* Also bump nettle to 2.7.1 (bug fixes)

cc:@loladiro