[REVIEW:3-5] fix download and unpack-sources for .xz source tarballs

[Petr Mladek]

Hi,

"the scripts "download" and "unpack-sources" needed some changes to work
with the newly generated .xz source tarballs, see
http://cgit.freedesktop.org/libreoffice/core/commit/?id=f780859b76263638b9227122e465b0c629c7ca3e

I would be nice to get this into 3-5 branch. It can't make things worse
because it just did not work. I wonder why nobody has complained about
it yet ;-)

Best Regards,
Petr