[tlbuild] xz 5.0.3 in TL

[Karl Berry]

xz 5.0.3 was just released.  We may as well update.  I committed the
sources into Build/source/extra/xz, or they are at
http://tukaani.org/xz.

Build instructions at http://tug.org/texlive/build.html#xz are unchanged.

It seems the timestamps went awry, or something, because for me it tried
to rerun aclocal and the like, but hopefully it will work out anyway.
At least on my platforms where I don't have autotools installed I still
get binaries.

I continue to await asymptote to start the real builds.  John expected
it to be ready some days ago :(.

Thanks,
karl