[MacPorts] #61450: GSL @2.6: build fails after Mac OS Big Sur update

[MacPorts]

#61450: GSL @2.6: build fails after Mac OS Big Sur update
---------------------+--------------------
  Reporter:  mmphys  |      Owner:  (none)
      Type:  defect  |     Status:  new
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:  2.6.4
Resolution:          |   Keywords:  bigsur
      Port:  gsl     |
---------------------+--------------------

Comment (by ryandesign):

 You have Xcode 12.2 and its copy of the macOS 11 SDK, but the gsl port is
 not configured to require Xcode; it uses the command line tools if
 available, and you apparently do not have a version of the command line
 tools that includes the macOS 11 SDK. Please update your command line
 tools to the Xcode 12.2 version.

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Ticket URL: <https://trac.macports.org/ticket/61450#comment:1>
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