Autogen: Skip generated files for compatibility with CMake 3.8
The change in !543 (merged) changes behavior of existing projects that may not expect AUTOGEN
on generated files and do not yet set SKIP_AUTOGEN
on them. Disable the behavior change for now to fix the regression for CMake 3.9. We can restore it later with a policy.
In order to keep the implementation and tests working, add an undocumented property we can use in the tests to enable the behavior before the policy is introduced.
Fixes: #17031 (closed)
Issue: #16186 (closed)