[MacPorts] #61541: gd2 2.3.0_0 +x11.darwin on Intel/BigSur: clang: error: invalid version number in 'MACOSX_DEPLOYMENT_TARGET=11.0'

[MacPorts]

#61541: gd2 2.3.0_0 +x11.darwin on Intel/BigSur: clang: error: invalid version
number in 'MACOSX_DEPLOYMENT_TARGET=11.0'
------------------------+--------------------
  Reporter:  shibukawa  |      Owner:  (none)
      Type:  defect     |     Status:  closed
  Priority:  Normal     |  Milestone:
 Component:  ports      |    Version:  2.6.4
Resolution:  invalid    |   Keywords:
      Port:             |
------------------------+--------------------

Comment (by jmroot):

 => On my console, I got warning about xcode-select --install, but this
 command dumps the following message and I could install other ports like
 nodejs14 (it was built from source), so this is not the cause I think.
 >
 > {{{
 > % sudo xcode-select --install
 > xcode-select: error: command line tools are already installed, use
 "Software Update" to install updates
 > }}}

 That means CLTs are installed but doesn't tell you if they are outdated.
 Software Update will often not update them because of an Apple bug:
 ProblemHotlist#reinstall-clt

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Ticket URL: <https://trac.macports.org/ticket/61541#comment:3>
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