Otherwise the "statistics-channels" option in doc/misc/options and
doc/man/named.conf.5in is marked as "not configured" (contrary to what
we have in release tarballs as they were build on a different image that
has libjson-c and libxml2 in it).
Caused by #4895 that made the option dependant on libjson-c or libxml2
presence in the build image.