RHEL should not try to install subman
cmake tries to to install subman even after its removal in http://github.com/ceph/ceph/commit/1bcc870305dbbe9b3b04278b545cce60aeb4d869, so the SRPM now fails to build on RHEL 8.
As a quick workaround, I changed this to "-DWITH_SUBMAN=OFF" in ceph.spec.
"git grep -i subman" shows several more things that need to be removed.
(If we ever bring subscription-manager integration back in the future, I recommend making it a completely separate package so it doesn't have to live in the Ceph tree.)