[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 kencu):

 what say we use {{{MacOSX.sdk}}} preferentially on BigSur and up?

 That will save a lot of heartburn with all the gcc ports, python, perl,
 and all the others that bake in a path to the SDK.

 And it's how Apple is going now anyway.

 And helps ports build when they need a newer SDK than the versioned one
 that matches their system.

 This was undesirable in the 10.4 to 10.13 era, but it may be time to do it
 now for 11.x and up.

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Ticket URL: <https://trac.macports.org/ticket/61866#comment:5>
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