[MacPorts] #63608: openmpi/mpich: add support for clang12, and clang13 too if possible

[MacPorts]

#63608: openmpi/mpich: add support for clang12, and clang13 too if possible
----------------------------+----------------------
  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):

 It looks like `openmpi` and `mpich` build successfully, against both Clang
 12 and 13.

 Next step is to fix the implicit function declarations.

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Ticket URL: <https://trac.macports.org/ticket/63608#comment:5>
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