gh-129666: Add C11/C++11 to docs and -pedantic-errors to GCC/clang test_c[pp]ext tests (GH-130686)

Lib/test/test_cppext/setup.py

@@ -18,6 +18,14 @@
         # a C++ extension using the Python C API does not emit C++ compiler
         # warnings
         '-Werror',
+
+        # Ask for strict(er) compliance with the standard.
+        '-pedantic-errors',
+
+        # But allow C++11 features for -std=C++03. We use:
+        # - `long long` (-Wno-c++11-long-long)
+        # - comma at end of `enum` lists (no narrower GCC option exists)
+        '-Wno-c++11-extensions',
     ]
 else:
     # MSVC compiler flags