4.13 Double-Colon Rules
Double-colon rules are rules written with `::' instead of
`:' after the target names. They are handled differently from
ordinary rules when the same target appears in more than one rule.
When a target appears in multiple rules, all the rules must be the same
type: all ordinary, or all double-colon. If they are double-colon, each
of them is independent of the others. Each double-colon rule's commands
are executed if the target is older than any prerequisites of that rule.
If there are no prerequisites for that rule, its commands are always
executed (even if the target already exists). This can result in
executing none, any, or all of the double-colon rules.
Double-colon rules with the same target are in fact completely separate
from one another. Each double-colon rule is processed individually, just
as rules with different targets are processed.
The double-colon rules for a target are executed in the order they appear
in the makefile. However, the cases where double-colon rules really make
sense are those where the order of executing the commands would not matter.
Double-colon rules are somewhat obscure and not often very useful; they
provide a mechanism for cases in which the method used to update a target
differs depending on which prerequisite files caused the update, and such
cases are rare.
Each double-colon rule should specify commands; if it does not, an
implicit rule will be used if one applies.
See section Using Implicit Rules.