[MacPorts] #57612: clang does not respect -syslibroot when linking

[MacPorts]

#57612: clang does not respect -syslibroot when linking
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  Reporter:  kencu   |      Owner:  (none)
      Type:  defect  |     Status:  new
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:
Resolution:          |   Keywords:
      Port:          |
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Comment (by jeremyhu):

 Well, 99% of the time, the value you pass to `-Wl,-syslibroot` is the same
 as what xcrun would choose or what MacPorts passes explicitly.

 I think the real problem here is that we are somehow in a state where
 MacPorts is trying to use an SDK that is different than what xcrun would
 be picking.  The logic in xcrun pretty much picks what one would expect in
 MacPorts (ie: it will pick the one matching the host version if present
 and then fall back on the latest one if not).

 Why are we in a situation where you are trying to use the 10.13 SDK on
 macOS 10.14?  Perhaps if I understood that, we could improve the logic in
 xcrun.

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Ticket URL: <https://trac.macports.org/ticket/57612#comment:28>
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