Update Autotools build for nettle library

* Per the nettle source, openssl is optionally linked against
  for performance testing only; this causes problems on RedHat/CentOS,
  so is disabled
* 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)
Merge pull request #7 from kmsquire/autotools_update

Update Autotools build for nettle library (fixes #5)