[MacPorts] #71776: ocrmypdf: move pngquant into a variant

[MacPorts]

#71776: ocrmypdf: move pngquant into a variant
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  Reporter:  barracuda156  |      Owner:  akierig
      Type:  defect        |     Status:  assigned
  Priority:  Normal        |  Milestone:
 Component:  ports         |    Version:  2.10.5
Resolution:                |   Keywords:
      Port:  ocrmypdf      |
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Comment (by i0ntempest):

 https://github.com/ocrmypdf/OCRmyPDF/blob/main/src/ocrmypdf/_exec/pngquant.py

 Should be moved to `depends_run` because it only calls the binary, but
 judging from this code, it's a hard dependency.

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Ticket URL: <https://trac.macports.org/ticket/71776#comment:2>
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