[cmake] feature=OFF has hierarchical priority over builtin_feature=ON

Fixes https://its.cern.ch/jira/browse/ROOT-10743

feature=ON can force-enable builtin_feature=OFF, but not the other way round, a warning is issued in that case now instead of silently enabling.

This prevents automatic enablings of features that the user can not disable unless he writes feature=OFF builtin_feature=OFF.

Author: ferdymercury