Use xz as the default tarball compression format

Use xz as the default tarball compression format

xz is quite a common format for software to be distributed nowadays
because of its size reductions.  With this patch, here is the result
on the master branch at db716e3b15912b7162def1faa704eb7823bbf34:

.rw-r--r--  406M sam   8 Nov 07:27   libabigail-2.2.tar.gz
.rw-r--r--  348M sam   8 Nov 07:26   libabigail-2.2.tar.xz

	* configure.ac: add dist-xz to AM_INIT_AUTOMAKE.
	* Makefile.am: adjust $(TARBALL) to new extension.
	* Makefile.am: pass XZ_OPT="-0" for distcheck-fast.

Signed-off-by: Sam James <sam@gentoo.org>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>