[MacPorts] #56300: Travis build failure of xorg-libXau

[MacPorts]

#56300: Travis build failure of xorg-libXau
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  Reporter:  ryandesign      |      Owner:  admin@…
      Type:  defect          |     Status:  new
  Priority:  Normal          |  Milestone:
 Component:  server/hosting  |    Version:
Resolution:                  |   Keywords:
      Port:                  |
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Comment (by mojca):

 See also #56110. We first need a minor change in `list-subports` to let
 Travis use it in a proper way.

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Ticket URL: <https://trac.macports.org/ticket/56300#comment:4>
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