'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'.