Fix _get_nacl() for libsodium >= 0.5.0
Fix _get_nacl() for libsodium >= 0.5.0

libsodium 0.5.0 resulted in a SONAME bump to libsodium.so.10, so
libnacl fails to load it. This fixes that issue, which will allow
libnacl to be built against libsodium >= 0.5.0.

johnttan/libnacl,coinkite/libnacl,saltstack/libnacl,cachedout/libnacl,RaetProtocol/libnacl,mindw/libnacl