ninja generator does not clean up Fortran .mod files
For a Fortran project with the "Unix Makefiles" generator, "make clean" wipes out all generated object files as well as the .mod files in the subdirectory "mod".
With the ninja generator (using https://github.com/Kitware/ninja/tree/features-for-fortran), it seems that "ninja clean" does wipe out the object files, but forgets about the .mod files!