package/xz-utils: does not require threads

Commit 799c12eb (xz-utils: needs threads) tried to fix an autobuild
failure by requiring threads. But xz-utils can be configured without
thread support (even though it is one of the most prominent
selling-points of the latest release!).

[Thomas: use --{enable,disable}-threads instead of
--enable-threads={yes,no}, to match what we generally do in most
autotools packages.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>