gh-116869: Fix test_cext for Free Threading

Author: vstinner

Lib/test/test_cext/setup.py

@@ -17,11 +17,14 @@
         # 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 be compatible with building
-        # with the -Werror=declaration-after-statement compiler flag.
-        '-Werror=declaration-after-statement',
     ]
+    # Free Threading doesn't build with -Werror=declaration-after-statement
+    if not sysconfig.get_config_var('Py_GIL_DISABLED'):
+        CFLAGS.append(
+            # gh-116869: The Python C API must be compatible with building
+            # with the -Werror=declaration-after-statement compiler flag.
+            '-Werror=declaration-after-statement',
+        )
 else:
     # Don't pass any compiler flag to MSVC
     CFLAGS = []