[MacPorts] #63676: gtk2 @2.24.33_0+quartz: build error on macOS 10.11: use of undeclared identifier 'kCAContentsFormatRGBA8Uint'

[MacPorts]

#63676: gtk2 @2.24.33_0+quartz: build error on macOS 10.11: use of undeclared
identifier 'kCAContentsFormatRGBA8Uint'
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  Reporter:  sambthompson  |      Owner:  mascguy
      Type:  defect        |     Status:  assigned
  Priority:  Normal        |  Milestone:
 Component:  ports         |    Version:  2.7.1
Resolution:                |   Keywords:  elcapitan
      Port:  gtk2          |
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Comment (by mascguy):

 Confirmed to be an issue on 10.11. Hopefully this is relatively easy to
 fix, but need to do some research first.

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Ticket URL: <https://trac.macports.org/ticket/63676#comment:4>
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