( Prerequisite Types

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 4.3 Types of Prerequisites
 There are actually two different types of prerequisites understood by
 GNU `make': normal prerequisites such as described in the previous
 section, and "order-only" prerequisites.  A normal prerequisite makes
 two statements: first, it imposes an order of execution of build
 commands: any commands necessary to build any of a target's
 prerequisites will be fully executed before any commands necessary to
 build the target.  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 out-of-date and must be rebuilt.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, however, you have a situation where you want to impose
 a specific ordering on the rules to be invoked _without_ forcing the
 target to be updated if one of those rules is executed.  In that case,
 you want to define "order-only" prerequisites.  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:
    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.  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 are a strict superset of the
 behavior of an order-only prerequisite).
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