Provide struct packing macros on GCC/clang out of the box

src/include/lwip/arch.h

@@ -209,54 +209,58 @@ extern "C" {
 #endif
 
 /** Packed structs support.
-  * Placed BEFORE declaration of a packed struct. \n
-  * For examples of packed struct declarations, see include/lwip/prot/ subfolder. \n
-  * Porting to GCC/clang: Nothing to do.
+  * Placed BEFORE declaration of a packed struct.\n
+  * For examples of packed struct declarations, see include/lwip/prot/ subfolder.\n
+  * A port to GCC/clang is included in lwIP, if you use these compilers there is nothing to do here.
   */
 #ifndef PACK_STRUCT_BEGIN
 #define PACK_STRUCT_BEGIN
 #endif /* PACK_STRUCT_BEGIN */
 
 /** Packed structs support.
-  * Placed AFTER declaration of a packed struct. \n
-  * For examples of packed struct declarations, see include/lwip/prot/ subfolder. \n
-  * Porting to GCC/clang: Nothing to do.
+  * Placed AFTER declaration of a packed struct.\n
+  * For examples of packed struct declarations, see include/lwip/prot/ subfolder.\n
+  * A port to GCC/clang is included in lwIP, if you use these compilers there is nothing to do here.
   */
 #ifndef PACK_STRUCT_END
 #define PACK_STRUCT_END
 #endif /* PACK_STRUCT_END */
 
 /** Packed structs support.
-  * Placed between end of declaration of a packed struct and trailing semicolon. \n
-  * For examples of packed struct declarations, see include/lwip/prot/ subfolder. \n
-  * Porting to GCC/clang: \#define PACK_STRUCT_STRUCT \_\_attribute\_\_((packed))
+  * Placed between end of declaration of a packed struct and trailing semicolon.\n
+  * For examples of packed struct declarations, see include/lwip/prot/ subfolder.\n
+  * A port to GCC/clang is included in lwIP, if you use these compilers there is nothing to do here.
   */
 #ifndef PACK_STRUCT_STRUCT
+#if defined(__GNUC__) || defined(__clang__)
+#define PACK_STRUCT_STRUCT __attribute__((packed))
+#else
 #define PACK_STRUCT_STRUCT
+#endif
 #endif /* PACK_STRUCT_STRUCT */
 
 /** Packed structs support.
-  * Wraps u32_t and u16_t members. \n
-  * For examples of packed struct declarations, see include/lwip/prot/ subfolder. \n
-  * Porting to GCC/clang: Nothing to do.
+  * Wraps u32_t and u16_t members.\n
+  * For examples of packed struct declarations, see include/lwip/prot/ subfolder.\n
+  * A port to GCC/clang is included in lwIP, if you use these compilers there is nothing to do here.
   */
 #ifndef PACK_STRUCT_FIELD
 #define PACK_STRUCT_FIELD(x) x
 #endif /* PACK_STRUCT_FIELD */
 
 /** Packed structs support.
-  * Wraps u8_t members, where some compilers warn that packing is not necessary. \n
-  * For examples of packed struct declarations, see include/lwip/prot/ subfolder. \n
-  * Porting to GCC/clang: Nothing to do.
+  * Wraps u8_t members, where some compilers warn that packing is not necessary.\n
+  * For examples of packed struct declarations, see include/lwip/prot/ subfolder.\n
+  * A port to GCC/clang is included in lwIP, if you use these compilers there is nothing to do here.
   */
 #ifndef PACK_STRUCT_FLD_8
 #define PACK_STRUCT_FLD_8(x) PACK_STRUCT_FIELD(x)
 #endif /* PACK_STRUCT_FLD_8 */
 
 /** Packed structs support.
-  * Wraps members that are packed structs themselves, where some compilers warn that packing is not necessary. \n
-  * For examples of packed struct declarations, see include/lwip/prot/ subfolder. \n
-  * Porting to GCC/clang: Nothing to do.
+  * Wraps members that are packed structs themselves, where some compilers warn that packing is not necessary.\n
+  * For examples of packed struct declarations, see include/lwip/prot/ subfolder.\n
+  * A port to GCC/clang is included in lwIP, if you use these compilers there is nothing to do here.
   */
 #ifndef PACK_STRUCT_FLD_S
 #define PACK_STRUCT_FLD_S(x) PACK_STRUCT_FIELD(x)