[MacPorts] #61866: base uses incorrect SDK for Big Sur 11.1 and Xcode 12.3

[MacPorts]

#61866: base uses incorrect SDK for Big Sur 11.1 and Xcode 12.3
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  Reporter:  mf2k    |      Owner:  (none)
      Type:  defect  |     Status:  new
  Priority:  Normal  |  Milestone:
 Component:  base    |    Version:  2.6.99
Resolution:          |   Keywords:
      Port:          |
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Comment (by jmroot):

 Replying to [comment:6 cjones051073]:
 > Personally, i think there is some merit to considering using by default
 the versionless sdk on big sur and newer.

 That will break things as soon as Xcode ships a macOS 12 SDK if not
 before. Almost everything checks for APIs at configure time, not at
 runtime.

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Ticket URL: <https://trac.macports.org/ticket/61866#comment:11>
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