gcc_release (build_gzip): Build xz tarball instead of bz2 tarball.

2017-05-18  Matthias Klose  <doko@ubuntu.com>

        * gcc_release (build_gzip): Build xz tarball instead of bz2 tarball.
        (build_diffs): Handle building diffs from either bz2 or xz tarballs,
        compress diffs using xz instead of bz2.
        (build_diff): Likewise.
        (upload_files): Check for *.xz files instead of *.bz2 files.
        (announce_snapshot): Announce xz tarball instead of bz2 tarball.
        (XZ): New definition.
        (<toplevel>): Look for both bz2 and xz compressed old tarballs.

From-SVN: r248251