dist: Use -9 instead of --best, and only for gzip, to pacify Alpine.

dist: Use -9 instead of --best, and only for gzip, to pacify Alpine.

From https://bugs.gnu.org/68151.

* lib/am/distdir.am (GZIP_ENV): Set to -9, not --best, since
Alpine does not support --best.
(distcheck): Don't use GZIP_ENV when invoking gzip for decompression,
since Alpine does not support -1..-9 with decompression.
* doc/automake.texi (The Types of Distributions): Document the change.
* NEWS: mention this. (En passant, add more change links.)