[MacPorts] #43216: gtk3: update to version 3.12.0

[MacPorts]

#43216: gtk3: update to version 3.12.0
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 Reporter:  devans@…  |      Owner:  cal@…
     Type:  defect    |     Status:  new
 Priority:  Normal    |  Milestone:
Component:  ports     |    Version:  2.2.1
 Keywords:            |       Port:  gtk3
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 I know you're working on this but attached is a patch that I have been
 using in testing of the GNOME 3.12.0 upgrades.
 Note in particular that it is necessary to explicitly disable the
 cloudprint backend to avoid opportunistic configuration that results in a
 circular dependency loop.

 Hope this is of some help.

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Ticket URL: <https://trac.macports.org/ticket/43216>
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