This is an attempt to address Issue #3326 by adding [*order-only*][1]
prerequsites of each build product on the directory where it is to go.
It is important that the prerequisites be order-only, since the
timestamp on a parent directory is not relevant to whether a product
is out of date; the parent directory merely needs to exist.
(This use case of generating target directories was provided as an
[example][2] of how order-only prequisites are used.)
[1]: http://www.gnu.org/software/make/manual/html_node/Prerequisite-Types.html
[2]: http://www.kolpackov.net/pipermail/notes/2004-January/000001.html
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