[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):

 `-Wl,-syslibroot` continues to do what it always has.  The issue is that
 passing it to xcrun does not stop xcrun from setting SDKROOT in the case
 where you are using an SDK to build.  The best way to do that is to use
 -isysroot.

 This behavior is really only relevant with Mojave because prior to Mojave,
 MacPorts users would install the DevSDK (headers at /).  With Mojave +, we
 use the SDK directly.  If an SDKROOT is not set (and not passed via
 canonical command line arguments), xcrun implicitly sets SDKROOT.  If you
 intend to set the SDKROOT when invoking clang, you should do that with
 `-isysroot` instead of `-Wl,-syslibroot.  The latter only passes that
 information to the linker whereas the former informs all stages of the
 build pipeline.

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Ticket URL: <https://trac.macports.org/ticket/57612#comment:26>
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