[MacPorts] #63085: clang-12: compiler-rt build sometimes fails parallel building

[MacPorts]

#63085: clang-12: compiler-rt build sometimes fails parallel building
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  Reporter:  blair     |      Owner:  kencu
      Type:  defect    |     Status:  closed
  Priority:  Normal    |  Milestone:
 Component:  ports     |    Version:
Resolution:  fixed     |   Keywords:
      Port:  clang-12  |
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Comment (by kencu):

 add_custom_target is the right fix I believe, but not the only fix.

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Ticket URL: <https://trac.macports.org/ticket/63085#comment:16>
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