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.