Pick gnutar for archive creation if available

Re-packing the archive using regular `tar` on OS X results in a lot of warnings when untaring on Linux:

```
tar: Ignoring unknown extended header keyword `SCHILY.dev'
tar: Ignoring unknown extended header keyword `SCHILY.ino'
tar: Ignoring unknown extended header keyword `SCHILY.nlink'
```

This change will use `gnutar` if it's available (e.g. via `brew install gnutar`).