rpmbuild w2.xzdio and multithreading

[Reindl Harald]

is there any way to set the two options below in .rpmmacros
to use multi-threading? the shell-script below works in case
of tar but XZ_OPT env is ignored by rpmbuild and seeking
two hours the internet don't give any answer

in case of tar --threads=0 improves performance dramatically
on a modern CPU most of the time with a lot of idle cores

%_binary_payload w2.xzdio
%_source_payload w2.xzdio
______________________________________________

#!/bin/bash
export XZ_OPT="-2 --threads=0"
/usr/bin/nice -n 19 /usr/bin/tar -p -cJf $1.tar.xz $1

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