[MacPorts] #45274: Run portbuild::build_main once for each build.target

[MacPorts]

#45274: Run portbuild::build_main once for each build.target
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  Reporter:  ryandesign@…  |      Owner:  macports-tickets@…
      Type:  enhancement   |     Status:  new
  Priority:  Normal        |  Milestone:
 Component:  base          |    Version:  2.3.99
Resolution:                |   Keywords:
      Port:                |
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Comment (by larryv@…):

 Strictly speaking, this change would penalize ports with correctly-written
 makefiles that would otherwise benefit from having their command-line
 goals executed in parallel, but the number of ports satisfying this
 condition might be insignificant. And in such cases it would be trivial to
 patch in a new phony target, dependent on the required targets, and use
 that as `${build.target}`.

 So this seems like a good idea to me.

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Ticket URL: <https://trac.macports.org/ticket/45274#comment:6>
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