+dh passes --parallel to dh_auto_* commands if it detects being run by the
+C<dpkg-buildpackage -jX> command, but a job server of the parent I<make>
+(presumably debian/rules) is not reachable. Nonetheless, it is highly
+recommended to pass --parallel/-j option to dh explicitly to indicate that a
+source package supports parallel building. See L<debhelper(7)/"BUILD SYSTEM
+OPTIONS"> for more information.
+