[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):

 > Where does that default isysroot come from? Because on 10.13, which has
 /usr/include, there is no default isysroot. It's only there on 10.14. But
 I don't know what puts it there.

 If -isysroot is on the command line, it is used. Otherwise ...
 If system headers are installed, they are used (no sysroot is set).
 Otherwise ...
 If xcrun finds a major-version matching SDK in the selected Xcode.app or
 in /Library/Developer/SDKs, it will use it.   Otherwise ...
 If xcrun finds a macosx SDK in the selected Xcode.app or in
 /Library/Developer/SDKs, it will use it.

 xcrun does not explicitly set the command line arguments.  It sets the
 SDKROOT envvar, which CFE honors as the default isysroot if set.

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Ticket URL: <https://trac.macports.org/ticket/57612#comment:18>
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