Comment 49 for bug 1710278

I have double-check in the buildlog and everything seems to indicate that multi-treading is turned off:

# buildlog
.....
 2689 dh_auto_configure -B build-singlethread -- \
 2690 --libdir=/usr/lib/x86_64-linux-gnu \
 2691 --sysconfdir=/etc/bind \
 2692 --with-python=python3 \
 2693 --localstatedir=/ \
=> 2694 --disable-threads \
 ...
 3908 Features disabled or unavailable on this platform:
=> 3909 Multiprocessing support (--enable-threads)
.....

Need to investigate more.