gh-116869: Fix test_cext on RHEL7

[vstinner]

Remove -std option from CC command line.

• Issue: Python 3.12.2 headers contain non-C90-compliant code #116869

[vstinner]

!buildbot s390x RHEL7

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @vstinner for commit 056b605 🤖

The command will test the builders whose names match following regular expression: s390x RHEL7

The builders matched are:

• s390x RHEL7 PR
• s390x RHEL7 LTO + PGO PR
• s390x RHEL7 LTO PR
• s390x RHEL7 Refleaks PR

[vstinner]

I fixed the first issue, but building for C++14 failed:

exit code: 1
  ╰─> [13 lines of output]
      CC env var: 'gcc -pthread'
      CFLAGS env var: <missing>
      CPPFLAGS env var: <missing>
      extra_compile_args: ['-Werror', '-DMODULE_NAME=_testcpp14ext', '-std=c++14']
      running bdist_wheel
      running build
      running build_ext
      building '_testcpp14ext' extension
      creating build
      creating build/temp.linux-s390x-cpython-313-pydebug
      gcc -pthread -fno-strict-overflow -Wsign-compare -g -Og -Wall -fPIC -I/home/dje/cpython-buildarea/pull_request.edelsohn-rhel-z/build/build/test_python_25841æ/tempcwd/env/include -I/home/dje/cpython-buildarea/pull_request.edelsohn-rhel-z/build/Include -I/home/dje/cpython-buildarea/pull_request.edelsohn-rhel-z/build -c extension.cpp -o build/temp.linux-s390x-cpython-313-pydebug/extension.o -Werror -DMODULE_NAME=_testcpp14ext -std=c++14
      gcc: error: unrecognized command line option ‘-std=c++14’
      error: command '/usr/bin/gcc' failed with exit code 1
      [end of output]

[vstinner]

!buildbot s390x RHEL7 PR

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @vstinner for commit fc70715 🤖

The command will test the builders whose names match following regular expression: s390x RHEL7 PR

The builders matched are:

• s390x RHEL7 PR