cppflags in test_c_unit_test is not supporting gcc compiler?

Bug Report

I newly forked the repo and ran the tests with runtests.py in the project root. Then, I am getting an error while running test_c_unit_test in test_external.py as following:

running build_ext
building 'test_capi' extension
gcc -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -arch x86_64 -g -mmacosx-version-min=10.10 -stdlib=libc++ -I../external/googletest -I../external/googletest/include -I/Users/terunobuinaba/OSS/mypy/venv/include -I/Library/Frameworks/Python.framework/Versions/3.9/include/python3.9 -c exc_ops.c -o build/temp.macosx-10.9-x86_64-3.9/exc_ops.o -Wno-unused-function -Wno-sign-compare
gcc: error: unrecognized command-line option '-stdlib=libc++'
error: command '/usr/local/bin/gcc' failed with exit code 1

mypyc/test/test_external.py:16 (TestExternal.test_c_unit_test)
self = <mypyc.test.test_external.TestExternal testMethod=test_c_unit_test>

    @unittest.skipIf(sys.platform.startswith("win"), "rt tests don't work on windows")
    def test_c_unit_test(self) -> None:
        """Run C unit tests in a subprocess."""
        # Build Google Test, the C++ framework we use for testing C code.
        # The source code for Google Test is copied to this repository.
        cppflags: List[str] = []
        env = os.environ.copy()
        if sys.platform == 'darwin':
            cppflags += ['-mmacosx-version-min=10.10', '-stdlib=libc++']
        env['CPPFLAGS'] = ' '.join(cppflags)
        subprocess.check_call(
            ['make', 'libgtest.a'],
            env=env,
            cwd=os.path.join(base_dir, 'mypyc', 'external', 'googletest', 'make'))
        # Build Python wrapper for C unit tests.
        env = os.environ.copy()
        env['CPPFLAGS'] = ' '.join(cppflags)
>       status = subprocess.check_call(
            [sys.executable, 'setup.py', 'build_ext', '--inplace'],
            env=env,
            cwd=os.path.join(base_dir, 'mypyc', 'lib-rt'))

mypyc/test/test_external.py:34: 

To Reproduce
After cloning and install mypy using pip install, I followed the instruction here

 pip3 install -r test-requirements.txt
 python2 -m pip install -U typing
 ./runtests.py

Also, I am not using clang but using gcc instead in my mac PC.

Expected Behavior

No error raised.

Actual Behavior

As the fourth line in the error above suggests: gcc: error: unrecognized command-line option '-stdlib=libc++', gcc supports only libstdc++. No option should be passed for gcc environments?

class TestExternal(unittest.TestCase):
    # TODO: Get this to work on Windows.
    # (Or don't. It is probably not a good use of time.)
    @unittest.skipIf(sys.platform.startswith("win"), "rt tests don't work on windows")
    def test_c_unit_test(self) -> None:
        """Run C unit tests in a subprocess."""
        # Build Google Test, the C++ framework we use for testing C code.
        # The source code for Google Test is copied to this repository.
        cppflags: List[str] = []
        env = os.environ.copy()
        if sys.platform == 'darwin':
            cppflags += ['-mmacosx-version-min=10.10', '-stdlib=libc++']    # passing cppflags parameter
        env['CPPFLAGS'] = ' '.join(cppflags)

I guess simply branching in cppflags += ['-mmacosx-version-min=10.10', '-stdlib=libc++'] between clang/gcc would work.

Your Environment

• Mypy version used: mypy 0.920+dev.492739692206f05347c6706cb982115446ba552a
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: Python 3.9.6
• Operating system and version: macOS Big Sur 11.6
• gcc version used: gcc version 10.2.0

P.S.
I am very new to open source contribution and this is actually the first issue I have made ever. I appreciate any comments, suggestions not only for the issue but also the manner of reporting the issue. Thank you for your guidance.

Commencez dans mypyc/test/test_external.py, au niveau de TestExternal.test_c_unit_test, et examinez comment runtests.py atteint ce test. Reproduisez la compilation sur macOS avec le compilateur signalé, puis vérifiez que la compilation du test unitaire C se termine sans transmettre de flags non pris en charge. La tâche est terminée lorsque le test fonctionne avec le compilateur signalé et que le comportement existant des tests sur macOS est préservé.