Enable -Wweak-vtables in clang
Read https://stackoverflow.com/a/23749273/11956287 for a full explanation, but basically this is important for headers like the interfaces/ and kdocumentviewer.h that are supposed to be used by third partyies where it may happen that wrong things happen because the whole class is inline in a header For the classes that are defined in the .cpp or are not installed headers this would not be necessary but it's not so hard to add a few default defined destructors, so do that :)