Remove warning with C++.

It'll will increase the size of the type a little bit.
TODO: Check compatibility between code using C def and code using C++ def

m-deque.h

@@ -46,6 +46,14 @@
 
 #define DEQUEUI_DEFAULT_SIZE  8
 
+/* C++ doesn't support flexible array within a structure.
+   Let's define at least one element. */
+#ifdef __cplusplus
+# define DEQUEI_MIN_ARRAY_SIZE 1
+#else
+# define DEQUEI_MIN_ARRAY_SIZE 
+#endif
+
 #define DEQUEI_CONTRACT(d) do {						\
     assert ((d) != NULL);						\
     assert ((d)->default_size >= DEQUEUI_DEFAULT_SIZE);			\
@@ -66,7 +74,7 @@
   typedef struct M_C(name, _node_s) {					\
     ILIST_INTERFACE(M_C(name, _node_list), struct M_C(name, _node_s));	\
     size_t size;							\
-    type  data[];							\
+    type  data[DEQUEI_MIN_ARRAY_SIZE];					\
   } node_t;								\
 									\
   ILIST_DEF(M_C(name, _node_list), node_t, (DEL(M_GET_FREE oplist)) )	\