[MacPorts] #69867: legacy-support headers should work with later SDK versions

[MacPorts]

#69867: legacy-support headers should work with later SDK versions
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 Reporter:  fhgwright  |      Owner:  fhgwright
     Type:  defect     |     Status:  assigned
 Priority:  Low        |  Milestone:
Component:  ports      |    Version:  2.9.3
 Keywords:             |       Port:  legacy-support
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 It's ordinarily legal (with some limitations) to build for a given OS
 target while using an SDK for a later OS version.  In some cases, the
 `legacy-support` headers may not work properly in this situation.  One
 known case is #69838.

 It would be desirable (though not high priority) to fix this.

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Ticket URL: <https://trac.macports.org/ticket/69867>
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