Commit 80dea664 broke .ui files with global includes that are not part
of the project, because we blindly added every file path that falls
out of 'uic -d' as dependency.
Introduce the extra compiler CONFIG flag dep_existing_only to bring
back the old behavior that ignores non-existent dependencies and set
it for uic.
Change-Id: I6eaa82817c932a98ebac6d08115a9815d4b9dd21
Fixes: QTBUG-78144
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
90779d96ad