How to make XZ_SETS compress using multiple threads?

[Paul Goyette <paul%whooppee.com@localhost>]

I see that we have an XZ_ARGS variable which we can force to define
with ``-V XZ_ARGS=-T6'' and that appears that it will add the "-T6"
argument when invoking xz to build sets.

However, this would break the evbmips Makefile, which already uses
XZ_ARGS to conditionally set several other flags.

Is there something equivalent to XZ_ARGS which can be set by a user
without interfering with the existing build mechanisms?



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