[MacPorts] #63085: clang-12: build from source fails on 11.4

[MacPorts]

#63085: clang-12: build from source fails on 11.4
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  Reporter:  blair     |      Owner:  (none)
      Type:  defect    |     Status:  new
  Priority:  Normal    |  Milestone:
 Component:  ports     |    Version:
Resolution:            |   Keywords:
      Port:  clang-12  |
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Comment (by kencu):

 hey blair, your log bit does not show what "error 1" is, but I'm sure I
 know what it is.

 try {{{use_parallel_build no}}}, which fixes the build of compiler-rt on
 some systems.

 for context as to why I don't want to put this into the portfile for
 everyone at present, see

 https://trac.macports.org/ticket/63038

 and in particular

 https://trac.macports.org/ticket/63038#comment:23

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Ticket URL: <https://trac.macports.org/ticket/63085#comment:1>
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