Idiom: no double-colon rules

Make has a special type of rule of the form target :: prerequisites, with the behaviour that all double-colon rules for a given target are executed if the target needs to be rebuilt. This style was popular for things like "all" and "clean" targets in the past, but it's not really necessary - see the "all" idiom above - and this means there's one fewer makeism you need to know about.