[MacPorts] #63608: openmpi/mpich: add support for clang12; evaluate clang13

[MacPorts]

#63608: openmpi/mpich: add support for clang12; evaluate clang13
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  Reporter:  mascguy        |      Owner:  mascguy
      Type:  enhancement    |     Status:  assigned
  Priority:  Normal         |  Milestone:
 Component:  ports          |    Version:
Resolution:                 |   Keywords:
      Port:  openmpi mpich  |
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Comment (by mascguy):

 Update: Still waiting to see whether `openmpi` and `mpich` build
 successfully, with Clang 12 and 13. But one thing that was immediately
 apparent, is the need to cleanup implicit function declarations.

 Shouldn't be a big deal, in principle. But definitely needs to be done
 first, particularly for ARM.

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Ticket URL: <https://trac.macports.org/ticket/63608#comment:3>
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