Install man pages when sphinx-build tool is missing
Only the 3f6cc254 commit which adds a CI check is meant to be backported.
Validation pipeline with globally set RUN_MAKE_INSTALL=1
and stress tests: https://gitlab.isc.org/isc-projects/bind9/-/pipelines/69886
Closes #2629 (closed)
Edited by Michał Kępień