XZ_THREADS and XZ_MEMLIMIT were introduced in dunfell.
[RP improved an original patch from Paul]
(From yocto-docs rev: 96defb66b775093b5270bd1ebad0461c2bba1e5b)
Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>