gh-116869: Build test_cext with -Werror=declaration-after-statement (#116990)

Make sure that the C API does not emit compiler warnings when built
with -Werror=declaration-after-statement.

Author: vstinner

Lib/test/test_cext/__init__.py

@@ -1,5 +1,7 @@
 # gh-116869: Build a basic C test extension to check that the Python C API
 # does not emit C compiler warnings.
+#
+# Python C API must build with -Werror=declaration-after-statement.
 
 import os.path
 import shutil

Lib/test/test_cext/setup.py

@@ -16,6 +16,10 @@
         # The purpose of test_cext extension is to check that building a C
         # extension using the Python C API does not emit C compiler warnings.
         '-Werror',
+
+        # gh-116869: The Python C API must build with
+        # -Werror=declaration-after-statement.
+        '-Werror=declaration-after-statement',
     ]
 else:
     # Don't pass any compiler flag to MSVC