[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):

 Replying to [comment:11 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.

 It will break all over the world though, as every other build system is
 using the current xcode against the latest MacOS.sdk, and so is cmake
 defaulting to that now.

 Headers are not a significant issue. It's the library symbols that might
 be found in the newer sdk, which should not be a significant issue for one
 system version (but could be...) and if we run into that, everyone using
 Xcode all over the place will also run into that, and it needs to be fixed
 for everyone anyway.

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Ticket URL: <https://trac.macports.org/ticket/61866#comment:12>
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