Only use DEB generator if CPACK_GENERATOR is unset

When trying to build a dmg package on an apple system the values set in
mk/macos/CMakeLists.txt are masked by those coded in the main CMakeLists file.
This change means the values are only set if they haven't been already (eg by
using the apple CMakeLists).

I feel it would probably be better to move this lot ot mk/linux/CMakeLists and
then conditionally import the correct file - I'm happy to redo the change that
way if there is agreement on that.