build system: Switch to xz compressed tarball

automake >= 1.11 is required now, but where do I specify it?

Author: scop

configure.ac

@@ -1,6 +1,6 @@
 AC_PREREQ([2.60])
 AC_INIT([bash-completion], [2.1])
-AM_INIT_AUTOMAKE([foreign dist-bzip2 no-dist-gzip -Wall -Werror])
+AM_INIT_AUTOMAKE([foreign dist-xz no-dist-gzip -Wall -Werror])
 AC_PROG_LN_S
 AC_PROG_MKDIR_P
 AC_SUBST(compatdir, $sysconfdir/bash_completion.d)