initialize DEBUG=0 and VERBOSE=0 by default
rename SILENT{MSG,CMD} to {MSG,CMD}
Only include *.d files if the specified operation is not "clean". Prior to this change when a user entered for example "make release; make clean; make clean" on the second "make clean" the *.d files would be generated and then immediately deleted by rm. By not including the *.d files if the operation is "clean" we avoid the redundant generation of the *.d files.