[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:  assigned
  Priority:  Normal    |  Milestone:
 Component:  ports     |    Version:
Resolution:            |   Keywords:
      Port:  clang-12  |
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Comment (by kencu):

 well now I am a bit confused, as all the Makefiles already have parallel
 building disabled, it appears:
 {{{
 # Allow only one "make -f Makefile2" at a time, but pass parallelism.
 .NOTPARALLEL:
 }}}

 At this moment, I don't understand exactly what "but pass parallelism"
 exactly means here.

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Ticket URL: <https://trac.macports.org/ticket/63085#comment:8>
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