[MacPorts] #51310: ImageMagick: update to 7.x

[MacPorts]

#51310: ImageMagick: update to 7.x
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  Reporter:  mopihopi     |      Owner:  ryandesign
      Type:  update       |     Status:  assigned
  Priority:  Normal       |  Milestone:
 Component:  ports        |    Version:
Resolution:               |   Keywords:
      Port:  ImageMagick  |
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Comment (by kencu):

 homebrew has had both 6 and 7 co-installing for years now. You might take
 a ppek at what they did for the builds. Here is 7.1:

 https://github.com/Homebrew/homebrew-core/blob/HEAD/Formula/imagemagick.rb

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Ticket URL: <https://trac.macports.org/ticket/51310#comment:40>
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