[MacPorts] #66378: llvm-13 fails to build as +universal on 10.8.5

[MacPorts]

#66378: llvm-13 fails to build as +universal on 10.8.5
---------------------------+--------------------------
  Reporter:  barracuda156  |      Owner:  catap
      Type:  defect        |     Status:  assigned
  Priority:  Normal        |  Milestone:
 Component:  ports         |    Version:  2.8.0
Resolution:                |   Keywords:  mountainlion
      Port:  llvm-13       |
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Comment (by barracuda156):

 Replying to [comment:1 kencu]:
 > So the proximate cause is that {{{/opt/local/lib/libstdc++.6.dylib}}}
 needs to be universal, and it is not universal, so the link fails.
 >
 > However, the real cause is less clear, as that file
 {{{/opt/local/lib/libstdc++.6.dylib}}} should not be called in to link
 code when {{{-stdlib=libc++}}}, which it appears to be.
 >
 > So where is that coming from? Are you using absolutely stock toolchains
 here, or are you using some alternative toolchain like the one from Iain's
 repo we recently added for testing with? I could imagine that some part of
 libtapi or ld64-something might be possibly calling in
 {{{/opt/local/lib/libstdc++.6.dylib}}}...

 On 10.8.5 everything is stock. And no `darwin-xtools` are not being used
 (not even installed there).

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Ticket URL: <https://trac.macports.org/ticket/66378#comment:2>
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