But the compiler generates different object code depending whether GetMTime() is merely inherited, versus whether it has a definition in vtkExtractCells. In the former case, it will need the symbol _ZN9vtkObject8GetMTimeEv, but in the latter case, it will need the symbol _ZN15vtkExtractCells8GetMTimeEv.
Your best bet is to grep your object files to see which one needs _ZN15vtkExtractCells8GetMTimeEv. By this, I mean simply do the following for the same list of object files that you are compiling into your module:
fgrep -l _ZN15vtkExtractCells8GetMTimeEv *.o