[MacPorts] #62124: gcc9 @9.3.0_4 gfortran fails with "ld: library not found for -lSystem" in Big Sur

[MacPorts]

#62124: gcc9 @9.3.0_4 gfortran fails with "ld: library not found for -lSystem" in
Big Sur
------------------------+--------------------
  Reporter:  cquike     |      Owner:  (none)
      Type:  defect     |     Status:  new
  Priority:  Normal     |  Milestone:
 Component:  ports      |    Version:  2.6.4
Resolution:             |   Keywords:  bigsur
      Port:  gcc8 gcc9  |
------------------------+--------------------

Comment (by cquike):

 I have attached the output of "gfortran-mp-9 -v -Wl,-v -Wa,-v -o hello
 hello.f". It indeed mentions
 /Library/Developer/CommandLineTools/SDKs/MacOSX11.0.sdk which is not
 present in my system:

 # ls -l /Library/Developer/CommandLineTools/SDKs/
 total 0
 lrwxr-xr-x  1 root  wheel   14 Jan 21 12:29 MacOSX.sdk -> MacOSX11.1.sdk
 drwxr-xr-x  8 root  wheel  256 Jul  9  2020 MacOSX10.15.sdk
 drwxr-xr-x  7 root  wheel  224 Nov 30 13:33 MacOSX11.1.sdk

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Ticket URL: <https://trac.macports.org/ticket/62124#comment:7>
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