[MacPorts] #58443: :error:build gcc6 cannot be built whilst libunwind-headers is installed.

[MacPorts]

#58443: :error:build gcc6 cannot be built whilst libunwind-headers is installed.
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  Reporter:  dbl001               |      Owner:  (none)
      Type:  defect               |     Status:  new
  Priority:  Normal               |  Milestone:
 Component:  ports                |    Version:  2.5.4
Resolution:                       |   Keywords:
      Port:  gcc6 gcc7 gcc8 gcc9  |
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Comment (by ryandesign):

 Please use `PortGroup conflicts_build 1.0` and `conflicts_build-append
 libunwind-headers` instead; it prints instructions for what the user
 should do.

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Ticket URL: <https://trac.macports.org/ticket/58443#comment:3>
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