patch 8.0.1709: some non-C89 code may slip through

Problem:    Some non-C89 code may slip through.
Solution:   Enforce C89 in configure. Fix detected problems. (James McCoy,
            closes #2735)

src/if_python3.c

@@ -815,7 +815,7 @@ python3_end(void)
 #endif
     if (Py_IsInitialized())
     {
-	// acquire lock before finalizing
+	/* acquire lock before finalizing */
 	PyGILState_Ensure();
 
 	Py_Finalize();