modify Makefile to contain clean target
modify Makefile to contain clean target

oxnz/design-patterns,PoisonBOx/design-patterns,oxnz/design-patterns,oxnz/design-patterns,PoisonBOx/design-patterns,oxnz/design-patterns,oxnz/design-patterns,oxnz/design-patterns,PoisonBOx/design-patterns,oxnz/design-patterns,PoisonBOx/design-patterns,oxnz/design-patterns,PoisonBOx/design-patterns,PoisonBOx/design-patterns,PoisonBOx/design-patterns,PoisonBOx/design-patterns