[MacPorts] #55660: unpaper @5.1 is outdated, 6.1 is there since 2014

[MacPorts]

#55660: unpaper @5.1 is outdated, 6.1 is there since 2014
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 Reporter:  detlevd            |      Owner:
     Type:  update             |     Status:  new
 Priority:  Normal             |  Milestone:
Component:  ports              |    Version:  2.4.2
 Keywords:  upgrade, haspatch  |       Port:  unpaper
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 Tools like 'ocrmypdf' require unpaper 6.1 or newer.
 GitHub has it.

 Adapting the Portfile to 6.1 (and adding the suggested compile flags) is
 straight forward.
 Tested on OSX 10.12.6 with clang 9.0.0 and Xcode 9.2

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Ticket URL: <https://trac.macports.org/ticket/55660>
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