[MacPorts] #64316: gcc9 @9.4.0_1 & gcc10 @10.3.0_1: build failure: Undefined symbols for architecture x86_64

[MacPorts]

#64316: gcc9 @9.4.0_1 & gcc10 @10.3.0_1: build failure: Undefined symbols for
architecture x86_64
-------------------------+--------------------
  Reporter:  i0ntempest  |      Owner:  (none)
      Type:  defect      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:  2.7.1
Resolution:              |   Keywords:
      Port:  gcc9 gcc10  |
-------------------------+--------------------

Comment (by EJFielding):

 Yes, I was not able to run `port upgrade outdated` yesterday on my Mac
 running 11.6.5 with Xcode 13.2.1. When it tried to rebuild `gcc10` it
 failed with the same errors. I tried `uninstall gcc10 libgcc10` and it
 told me this would break my `openmpi-default` port that was built with
 `+gcc10`. I finally went ahead and uninstalled `gcc9 libgcc9 gcc10
 libgcc10`. Then I uninstalled and reinstalled `openmpi-default`. I also
 had to do that with `OpenBLAS`. Those take a long time to rebuild, but now
 they are both successfully installed with `+gcc11`. As I understand it,
 they are not going to fix gcc9 or gcc10 to compile on recent versions of
 Xcode.

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Ticket URL: <https://trac.macports.org/ticket/64316#comment:13>
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