cmake: Convert no source/build dir error to warning

This temporarily restores the previous behavior that allowed the cmake command line to have no source or build dir specified when at least one other option was given. The check for this that was introduced in 3.13.3 issued a fatal error, but this broke some user scripts that were relying on the (undocumented and unsupported) behavior. This MR downgrades that error to a warning with additional advisory comments.

The first commit should be reverted on master after this change is merged. This will restore the error for the check for the 3.14 release series. The second commit should not be reverted, it adds 3.13.4 release notes that will need to remain going forward.

Fixes: #18817 (closed)