gh-141563: Fix test_cppext on macOS (#144685)

Don't test internal header files including mimalloc on macOS since
mimalloc emits compiler warnings:

  In file included from extension.cpp:21:
  In file included from Include/internal/pycore_backoff.h:15:
  In file included from Include/internal/pycore_interp_structs.h:15:
  In file included from Include/internal/pycore_tstate.h:14:
  In file included from Include/internal/pycore_mimalloc.h:43:
  Include/internal/mimalloc/mimalloc.h:464:85: error: defaulted
  function definitions are a C++11 extension
  [-Werror,-Wc++11-extensions]
    mi_stl_allocator() mi_attr_noexcept = default;
                                                                                      ^
  Include/internal/mimalloc/mimalloc.h:465:85: error: defaulted
  function definitions are a C++11 extension
  [-Werror,-Wc++11-extensions]
    mi_stl_allocator(const mi_stl_allocator&) mi_attr_noexcept = default;

Log also CXX and CXXFLAGS env vars in test_cppext. Log also CPPFLAGS
in test_cext.

Author: vstinner

Lib/test/test_cext/setup.py

@@ -113,7 +113,7 @@ def main():
             print(f"Add PCbuild directory: {pcbuild}")
 
     # Display information to help debugging
-    for env_name in ('CC', 'CFLAGS'):
+    for env_name in ('CC', 'CFLAGS', 'CPPFLAGS'):
         if env_name in os.environ:
             print(f"{env_name} env var: {os.environ[env_name]!r}")
         else:

Lib/test/test_cppext/extension.cpp

@@ -18,8 +18,9 @@
 #  include "internal/pycore_frame.h"
    // mimalloc emits many compiler warnings when Python is built in debug
    // mode (when MI_DEBUG is not zero).
-   // mimalloc emits compiler warnings when Python is built on Windows.
-#  if !defined(Py_DEBUG) && !defined(MS_WINDOWS)
+   // mimalloc emits compiler warnings when Python is built on Windows
+   // and macOS.
+#  if !defined(Py_DEBUG) && !defined(MS_WINDOWS) && !defined(__APPLE__)
 #    include "internal/pycore_backoff.h"
 #    include "internal/pycore_cell.h"
 #  endif

Lib/test/test_cppext/setup.py

@@ -101,7 +101,7 @@ def main():
             print(f"Add PCbuild directory: {pcbuild}")
 
     # Display information to help debugging
-    for env_name in ('CC', 'CFLAGS', 'CPPFLAGS'):
+    for env_name in ('CC', 'CXX', 'CFLAGS', 'CPPFLAGS', 'CXXFLAGS'):
         if env_name in os.environ:
             print(f"{env_name} env var: {os.environ[env_name]!r}")
         else: