Added -x option to makerel to produce .xz tarballs

With the default of -6 produced much smaller tarball than
.gz (7z produced) and .bz2

  -rw-r--r--  1 bingos bingos 13601234 Sep 29 20:07 perl-5.21.5.tar.bz2
  -rw-r--r--  1 bingos bingos 15699515 Sep 29 20:07 perl-5.21.5.tar.gz
  -rw-r--r--  1 bingos bingos 10982580 Sep 29 20:08 perl-5.21.5.tar.xz

It's an option if we ever need it.