What is double colon target in Makefiles?

Double colon target - can exist in a makefile more than once with different dependencies and target rules. Another effect is that they are always executed so they work nicely for directory traversal targets where the target name is a directory which is always up to date. Finally once you define a target name with a double colon you can not have the same name with a single colon.

directory::
  cd $@ && "$(MAKE)"