[MacPorts] #53515: mapnik: Update to 3.0.12

[MacPorts]

#53515: mapnik: Update to 3.0.12
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  Reporter:  ryandesign  |      Owner:  macports@…
      Type:  update      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:
Resolution:              |   Keywords:
      Port:  mapnik      |
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Comment (by ryandesign):

 I've updated icu to 58.2 (see #49614), which breaks mapnik and all other
 ports linking with icu until they're rebuilt. I've revbumped the other
 ports to rebuild them, but unfortunately, with this update, mapnik 3.0.10
 fails to build. I've [https://github.com/mapnik/mapnik/issues/3654
 reported it upstream]. I tried updating to 3.0.12 (the latest version
 according to the project's download web page) and 3.0.13 (the latest
 version according to the project's GitHub project), and they still fail
 the same way. I'll attach my in-progress diffs.

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Ticket URL: <https://trac.macports.org/ticket/53515#comment:1>
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