[MacPorts] #56996: libgcc45 @4.5.4_15: fails to upgrade on macOS 10.12.6

[MacPorts]

#56996: libgcc45 @4.5.4_15: fails to upgrade on macOS 10.12.6
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  Reporter:  mopihopi  |      Owner:  (none)
      Type:  defect    |     Status:  new
  Priority:  Normal    |  Milestone:
 Component:  ports     |    Version:
Resolution:            |   Keywords:
      Port:  libgcc45  |
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Comment (by ryandesign):

 As far as I remember, gcc prior to version 5 doesn't build with Xcode 9 or
 later, regardless of OS version, and we should update the error message to
 reflect that. If you must use gcc, please use a newer one.

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Ticket URL: <https://trac.macports.org/ticket/56996#comment:2>
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