dist: allow running several compressors in parallel.

dist: allow running several compressors in parallel.

* lib/am/distdir.am (am__post_remove_distdir): New internal
variable.
(DIST_TARGETS): New variable, set to list of chosen distribution
formats.  Order formats by expected duration, slowest first, for
better parallelism.
(dist-gzip, dist-bzip2, dist-lzma, dist-xz, dist-tarZ)
(dist-shar, dist-zip): Use $(am__post_remove_distdir).
(dist, dist-all): Do not depend on distdir.  Instead of
replicating each compression command, use a recursive invocation
to allow running all $(DIST_TARGETS) in parallel.
* NEWS: Update.
Report by Peter Breitenlohner.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>