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

[MacPorts]

#71776: ocrmypdf: move pngquant into a variant
--------------------------+----------------------
 Reporter:  barracuda156  |      Owner:  akierig
     Type:  defect        |     Status:  assigned
 Priority:  Normal        |  Milestone:
Component:  ports         |    Version:  2.10.5
 Keywords:                |       Port:  ocrmypdf
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 `pngquant` pulls in a dependency on rust, which is undesirable (huge to
 build, broken on legacy systems). Please move it into a variant, so that
 the port can be built without rust. Variant can be set as default one
 wherever needed version of rust is supported, so users won't lose any
 functionality.

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