When in automatic dependency mode, never delete any intermediate

When in automatic dependency mode, never delete any intermediate
files automatically.  Otherwise, the following could happen: When
starting from a clean source tree, the first build would delete the
intermediate file, but also create the dependency file, which
mentions the intermediate file, thus making it non-intermediate.
The second build will then need to rebuild the now non-intermediate
missing file.  So the second build will do work even though nothing
had changed.  One place where this happens is the .c -> .o -> .so
chain for some contrib modules.