[MacPorts] #56511: gcc49 @4.9.4_2: build fails with Xcode 9 on macOS 10.12

[MacPorts]

#56511: gcc49 @4.9.4_2: build fails with Xcode 9 on macOS 10.12
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  Reporter:  mopihopi  |      Owner:  (none)
      Type:  defect    |     Status:  new
  Priority:  Normal    |  Milestone:
 Component:  ports     |    Version:
Resolution:            |   Keywords:
      Port:  gcc49     |
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Comment (by kencu):

 I tried to see if I might build `gcc49` using a different compiler, like
 `gcc6`, but in the end it failed as well on `darwin 17`. It still pulls in
 some of the current macOS SDK even with these other compilers, which is
 the basis of the build issue.

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Ticket URL: <https://trac.macports.org/ticket/56511#comment:10>
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