[MacPorts] #70352: gtk3 fails to pass legacy-support include flag

[MacPorts]

#70352: gtk3 fails to pass legacy-support include flag
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  Reporter:  barracuda156          |      Owner:  mascguy
      Type:  defect                |     Status:  assigned
  Priority:  Normal                |  Milestone:
 Component:  ports                 |    Version:  2.9.3
Resolution:                        |   Keywords:
      Port:  gtk3, legacy-support  |
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Comment (by barracuda156):

 Replying to [comment:1 fhgwright]:
 > I believe the `legacysupport` PortGroup sets up the proper flags for
 both the include and the library (and the dependency).  If you don't use
 the PortGroup, you'll need to do that "manually".
 >
 > This isn't an issue with `legacy-support` itself.

 PG is included, but flag is not passed. I think the build system discards
 our flags somewhere.

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Ticket URL: <https://trac.macports.org/ticket/70352#comment:3>
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