[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):

 Building against the 11.0 SDK if available is not wrong. We want to build
 binaries that will run on any macOS 11 release. If the 11.0 SDK is present
 as a directory but not usable for some reason, I guess we need some more
 checks to make sure an SDK is OK before using it.

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Ticket URL: <https://trac.macports.org/ticket/61866#comment:10>
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