[MacPorts] #55281: gmp: compile error on High Sierra

[MacPorts]

#55281: gmp: compile error on High Sierra
---------------------+---------------------------------
  Reporter:  mluzum  |      Owner:  MarcusCalhoun-Lopez
      Type:  defect  |     Status:  assigned
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:  2.4.2
Resolution:          |   Keywords:
      Port:  gmp     |
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Comment (by kencu):

 Hacking Availability.h and putting a copy in `/usr/local/include` or
 `/opt/local/include/` was something that was recommended a couple of years
 back to fix some temporary hiccup in Apple's header -- it worked to get
 past the issue of the moment -- but it nicely illustrates the down-the-
 road, hard-to-debug nonsense that can happen later on when you forget
 about that. There must have been a dozen tickets so far about that exact
 same issue, most often manifesting as `SWIFT_UNAVAILABLE`.

 Glad you sorted it out.

--
Ticket URL: <https://trac.macports.org/ticket/55281#comment:6>
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