Build statically salt against libsodium

Since libsodium is not widely deployed on the systems right now, it useful to just embed libsodium in the sources and build statically against it. Also since salt is now built statically against libsodium it's easier to deply an erlang release.

For now the patch is only building libsodium on UNIX (and cygwin probably), build with MVS is not yet supported.