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

[MacPorts]

#70352: gtk3 fails to pass legacy-support include flag
--------------------------+----------------------------------
 Reporter:  barracuda156  |      Owner:  mascguy
     Type:  defect        |     Status:  assigned
 Priority:  Normal        |  Milestone:
Component:  ports         |    Version:  2.9.3
 Keywords:                |       Port:  gtk3, legacy-support
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 The port uses `legacysupport`, but the build does not add needed include
 flags.

 Manually adding `configure.cppflags-append
 -I${prefix}/include/LegacySupport` works, but I am not sure this is the
 best solution.

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