Regarding https://debbugs.gnu.org/cgi/bugreport.cgi?bug=62069.
Given Kamila's point about reproducibility, and the xz version dependence of the feature, I don't think we should change automake's default of unthreaded. Thus I think the only change called for here is to update the doc to mention that -T0 (or another value) can presumably be added to XZ_OPT if a maintainer wishes to make thread-dependent, and xz-version-dependent, tarballs. I added that as below. Closing. --thanks, karl.
