Threaded version of TOOL_XZ ?

From recent investigations into PR install/54844, I have noted that we
don't have a useful build tool that can both

 	1) compress much better/smaller than gzip, and
 	2) use multiple threads to do parallel compressions
 	   to reduce run-time.

Using TOOL_XZ instead of TOOL_GZ satisfies requirement #1, and use of
(unsupported) TOOL_PIGZ satisfies #2, but neither option can meet
both.  TOOL_XZ is explicitly built without thread support, and simply
modifying its Makefile to enable_threads=yes doesn't build.

FWIW, on my 8core/16thread Intel CPU, PIGZ compression takes just a
fraction of the time GZ takes to build a debug set (saving  more than
20 minutes from a ``build.sh -J7 release'';  yet the resulting sets
files from PIGZ are 50% to 100% larger than those from XZ.

So, it would seem reasonable to provide an option to build TOOL_XZ
with threads enabled, even if only through a hidden option (and maybe
eveen useful only in build.sh's -E expert mode).

Suggestion?  Comments?  Patches? :-)


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