'Setup.hs sdist': invoke 'tar' with '--format ustar' if possible.

Fixes #1901. Some variants of `tar` don't support the `--format` option, so we have to do feature detection.

Note that 'cabal sdist' (as opposed to `Setup.hs sdist`) uses the built-in tar.gz archiver (`Distribution.Client.Tar`) and doesn't invoke `tar` or `gzip`.