[MacPorts] #55949: libpng.pc should have zlib in Requires.private

[MacPorts]

#55949: libpng.pc should have zlib in Requires.private
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  Reporter:  jmroot  |      Owner:  ryandesign
      Type:  defect  |     Status:  new
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:
Resolution:          |   Keywords:
      Port:  libpng  |
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Comment (by ryandesign):

 Well I've been wondering about that. But it's an upstream issue, right?

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Ticket URL: <https://trac.macports.org/ticket/55949#comment:1>
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