( Search Algorithm

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 4.5.3 How Directory Searches are Performed
 When a prerequisite is found through directory search, regardless of
 type (general or selective), the pathname located may not be the one
 that `make' actually provides you in the prerequisite list.  Sometimes
 the path discovered through directory search is thrown away.
    The algorithm `make' uses to decide whether to keep or abandon a
 path found via directory search is as follows:
   1. If a target file does not exist at the path specified in the
      makefile, directory search is performed.
   2. If the directory search is successful, that path is kept and this
      file is tentatively stored as the target.
   3. All prerequisites of this target are examined using this same
   4. After processing the prerequisites, the target may or may not need
      to be rebuilt:
        a. If the target does _not_ need to be rebuilt, the path to the
           file found during directory search is used for any
           prerequisite lists which contain this target.  In short, if
           `make' doesn't need to rebuild the target then you use the
           path found via directory search.
        b. If the target _does_ need to be rebuilt (is out-of-date), the
           pathname found during directory search is _thrown away_, and
           the target is rebuilt using the file name specified in the
           makefile.  In short, if `make' must rebuild, then the target
           is rebuilt locally, not in the directory found via directory
    This algorithm may seem complex, but in practice it is quite often
 exactly what you want.
    Other versions of `make' use a simpler algorithm: if the file does
 not exist, and it is found via directory search, then that pathname is
 always used whether or not the target needs to be built.  Thus, if the
 target is rebuilt it is created at the pathname discovered during
 directory search.
    If, in fact, this is the behavior you want for some or all of your
 directories, you can use the `GPATH' variable to indicate this to
    `GPATH' has the same syntax and format as `VPATH' (that is, a space-
 or colon-delimited list of pathnames).  If an out-of-date target is
 found by directory search in a directory that also appears in `GPATH',
 then that pathname is not thrown away.  The target is rebuilt using the
 expanded path.
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