[make] - GNU make 4.2 Types of Prerequisites

There are two different types of prerequisites understood by GNU make: normal prerequisites, described in the previous section, and order-only prerequisites.

A normal prerequisite makes two statements: first, it imposes an order in which recipes will be invoked: the recipes
for all prerequisites of a target will be completed before the recipe for the target is started.
Second, it imposes a dependency relationship: if any prerequisite is newer than the target,
then the target is considered out-of-date and must be rebuilt.
Normally, this is exactly what you want: if a target’s prerequisite is updated, then the
target should also be updated.

 

Occasionally you may want to ensure that a prerequisite is built before a target, but
without forcing the target to be updated if the prerequisite is updated. Order-only prerequisites are used to create this type of relationship. Order-only prerequisites can be specified
by placing a pipe symbol (|) in the prerequisites list: any prerequisites to the left of the
pipe symbol are normal; any prerequisites to the right are order-only:
targets : normal-prerequisites | order-only-prerequisites
The normal prerequisites section may of course be empty. Also, you may still declare
multiple lines of prerequisites for the same target: they are appended appropriately (normal
prerequisites are appended to the list of normal prerequisites; order-only prerequisites are
appended to the list of order-only prerequisites). Note that if you declare the same file to
be both a normal and an order-only prerequisite, the normal prerequisite takes precedence
(since they have a strict superset of the behavior of an order-only prerequisite).
Order-only prerequisites are never checked when determining if the target is out of date;
even order-only prerequisites marked as phony (see Section 4.5 [Phony Targets], page 31)
will not cause the target to be rebuilt.
Consider an example where your targets are to be placed in a separate directory, and that
directory might not exist before make is run. In this situation, you want the directory to
be created before any targets are placed into it but, because the timestamps on directories
change whenever a file is added, removed, or renamed, we certainly don’t want to rebuild
all the targets whenever the directory’s timestamp changes. One way to manage this is with
order-only prerequisites: make the directory an order-only prerequisite on all the targets: