Usef __BYTE_ORDER__ instead of __BYTE_ORDER (#513)

__BYTE_ORDER__ is common predefined macro available on at least gcc and clang.
__BYTE_ORDER is macro defined in platform specific headers.

Author: kulikjak

msgpack/sysdep.h

@@ -61,14 +61,12 @@ typedef unsigned int _msgpack_atomic_counter_t;
 #endif
 #endif
 
-#else
-#include <arpa/inet.h>  /* __BYTE_ORDER */
 #endif
 
 #if !defined(__LITTLE_ENDIAN__) && !defined(__BIG_ENDIAN__)
-#if __BYTE_ORDER == __LITTLE_ENDIAN
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
 #define __LITTLE_ENDIAN__
-#elif __BYTE_ORDER == __BIG_ENDIAN
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
 #define __BIG_ENDIAN__
 #elif _WIN32
 #define __LITTLE_ENDIAN__