Add xz compression support.
* NEWS: Mention xz, as well as lzma (xz will displace lzma).
* automake.in (handle_dist): Recognize dist-xz.
(make_paragraphs): Map XZ to dist-xz.
* doc/automake.texi (Dist): Add dist-xz.
(Options): Likewise.
* lib/Automake/Options.pm (_process_option_list): Accept
dist-xz.
* lib/am/distdir.am (dist-xz): New rule.
(dist dist-all): Add command to create an xz-compressed tarball.
(distcheck): Handle xz-compressed tarballs just like the others.
* tests/xz.test: New file, based on nogzip.test.
* tests/Makefile.am (TESTS): Add xz.test.
Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>