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.

Beginne in mypyc/test/test_external.py bei TestExternal.test_c_unit_test und prüfe, wie runtests.py zu diesem Test gelangt. Reproduziere den Build auf macOS mit dem gemeldeten Compiler und verifiziere anschließend, dass der Build des C-Unit-Tests abgeschlossen wird, ohne nicht unterstützte Flags zu übergeben. Die Aufgabe ist abgeschlossen, wenn der Test mit dem gemeldeten Compiler funktioniert und das bestehende macOS-Testverhalten erhalten bleibt.