gh-91321: Add _testcppext C++ extension (#32175)

Build a basic C++ test extension to check that the Python C API is
compatible with C++ and does not emit C++ compiler warnings.

* Add Modules/_testcppext.cpp: C++ extension
* Add Lib/test/test_cppext.py: test building the C++ extension.

Lib/test/test_cppext.py

@@ -0,0 +1,79 @@
+# gh-91321: Build a basic C++ test extension to check that the Python C API is
+# compatible with C++ and does not emit C++ compiler warnings.
+import os
+import sys
+import unittest
+import warnings
+from test import support
+from test.support import os_helper
+
+with warnings.catch_warnings():
+    warnings.simplefilter('ignore', DeprecationWarning)
+    from distutils.core import setup, Extension
+    import distutils.sysconfig
+
+
+MS_WINDOWS = (sys.platform == 'win32')
+
+
+SOURCE = support.findfile('_testcppext.cpp')
+if not MS_WINDOWS:
+    # C++ compiler flags for GCC and clang
+    CPPFLAGS = [
+        # Python currently targets C++11
+        '-std=c++11',
+        # gh-91321: The purpose of _testcppext extension is to check that building
+        # a C++ extension using the Python C API does not emit C++ compiler
+        # warnings
+        '-Werror',
+    ]
+else:
+    # Don't pass any compiler flag to MSVC
+    CPPFLAGS = []
+
+
+class TestCPPExt(unittest.TestCase):
+    def build(self):
+        cpp_ext = Extension(
+            '_testcppext',
+            sources=[SOURCE],
+            language='c++',
+            extra_compile_args=CPPFLAGS)
+
+        try:
+            try:
+                with (support.captured_stdout() as stdout,
+                      support.swap_attr(sys, 'argv', ['setup.py', 'build_ext'])):
+                    setup(name="_testcppext", ext_modules=[cpp_ext])
+                    return
+            except:
+                # Show output on error
+                print()
+                print(stdout.getvalue())
+                raise
+        except SystemExit:
+            self.fail("Build failed")
+
+    # With MSVC, the linker fails with: cannot open file 'python311.lib'
+    # https://github.com/python/cpython/pull/32175#issuecomment-1111175897
+    @unittest.skipIf(MS_WINDOWS, 'test fails on Windows')
+    def test_build(self):
+        # save/restore os.environ
+        def restore_env(old_env):
+            os.environ.clear()
+            os.environ.update(old_env)
+        self.addCleanup(restore_env, dict(os.environ))
+
+        def restore_sysconfig_vars(old_config_vars):
+            distutils.sysconfig._config_vars.clear()
+            distutils.sysconfig._config_vars.update(old_config_vars)
+        self.addCleanup(restore_sysconfig_vars,
+                        dict(distutils.sysconfig._config_vars))
+
+        # Build in a temporary directory
+        with os_helper.temp_cwd():
+            self.build()
+
+
+if __name__ == "__main__":
+    unittest.main()