4.7 Rules without Commands or Prerequisites
If a rule has no prerequisites or commands, and the target of the rule
is a nonexistent file, then
make imagines this target to have
been updated whenever its rule is run. This implies that all targets
depending on this one will always have their commands run.
An example will illustrate this:
Here the target `FORCE' satisfies the special conditions, so the
target `clean' that depends on it is forced to run its commands.
There is nothing special about the name `FORCE', but that is one name
commonly used this way.
As you can see, using `FORCE' this way has the same results as using
Using `.PHONY' is more explicit and more efficient. However,
other versions of
make do not support `.PHONY'; thus
`FORCE' appears in many makefiles. See section 4.6 Phony Targets.