The dependency to libevent-pthread came in version 28.7. The changelog doesn't
directly say it, but it is part of the "drop support for libevent < 2.1.4",
ref. OwnTone PR #1574: "Support for libevent < 2.1.4 has been removed, and
libevent pthreads is now a hard requirement." The configure option
--without-libevent_pthreads also no longer exists.
Signed-off-by: Espen Jürgensen <espenjurgensen+openwrt@gmail.com>