build system: fix distribution tarball generation
build system: fix distribution tarball generation

* generate distribution tarball in *.tar.xz format, rather
  than *.tar.gz (the former is used at gnome.org)
* leaving 'tar-ustar' in place, it forces posix tar
  (which supports path name lengths > 99 chars)

GNOME/evolution-kolab,GNOME/evolution-kolab