Mention that Linux builds now do not enable threads by default.
Mention that Linux builds now do not enable threads by default.

pecharmin/bind9,pecharmin/bind9,each/bind9-collab,pecharmin/bind9,each/bind9-collab,each/bind9-collab,pecharmin/bind9,each/bind9-collab,pecharmin/bind9,each/bind9-collab,pecharmin/bind9,each/bind9-collab