Generalize the DEPFILE support check for the add_custom_command

We have multiple places where we check if DEPFILE is supported by the add_custom_command and provide similar workarounds in opposite case.

The DEPFILE support expands in newer CMake versions, so it makes sense to generalize the check and the workaround in common function, e.g. qt_add_custom_command, to make the use of DEPFILE consistent.