Do not include dependency files if make command goals contain "clean".

Do not include dependency files if make command goals contain "clean".

Calling "make clean2" was particularly annoying because it first regenerated the
dependency files it is supposed to remove.

Unfortunately, this patch breaks multiple goals if one of them matches "clean".
For example "make clean all" does no longer work. But as this can easily be worked
around by calling "make clean && make all", we don't care.