Runtime linker error in some builds --without-system-gcc

[mkoeppe]

This is only a minor problem because we really only need our own gcc build when system GCC is 10.x.

tox -e docker-debian-buster-standard-python2-gcc_spkg (​https://github.com/mkoeppe/sage/runs/661787508):

  [dochtml]     File "sage/matrix/args.pyx", line 23, in init sage.matrix.args (build/cythonized/sage/matrix/args.c:21224)
  [dochtml]     File "/sage/local/lib/python2.7/site-packages/sage/matrix/matrix_space.py", line 46, in <module>
  [dochtml]       from . import matrix_modn_sparse
  [dochtml]     File "sage/matrix/matrix_integer_sparse.pxd", line 5, in init sage.matrix.matrix_modn_sparse (build/cythonized/sage/matrix/matrix_modn_sparse.cpp:16301)
  [dochtml]   ImportError: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.26' not found (required by /sage/local/lib/python2.7/site-packages/sage/matrix/matrix_integer_sparse.so)
  [dochtml] Full log file: logs/dochtml.log

Also:

• tox -e docker-debian-buster-standard-gcc-spkg (​https://github.com/mkoeppe/sage/runs/743789697)
• tox -e docker-ubuntu-bionic-standard-gcc-spkg

(​https://github.com/mkoeppe/sage/runs/743789609)

[mkoeppe]

Sage development has entered the release candidate phase for 9.3. Setting a new milestone for this ticket based on a cursory review of ticket status, priority, and last modification date.