[ng] dist: refactor handling of dist formats a little

[ng] dist: refactor handling of dist formats a little

* automake.in (handle_dist): Define list of default dist formats in the
new internal make variable 'am.dist.formats'.  Related adjustments.
(preprocess_file): Drop the following now-unused transforms: XZ, LZMA,
LZIP, BZIP2, COMPRESS, GZIP, SHAR, ZIP.
* lib/am/distdir.am: Drop any use of the removed transforms, throughout.
(DIST_TARGETS): Redefine in function of '$(am.dist.formats)'.
(.PHONY): Update in function of '$(am.dist.formats)'.
(am.dist.ext.gzip, am.dist.ext.bzip2, am.dist.ext.xz, am.dist.ext.lzip,
am.dist.ext.tarZ, am.dist.ext.shar, am.dist.ext.zip): New internal
variables.
(DIST_ARCHIVES): Redefine in function of them and of '$(am.dist.formats)'.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>