Check if macros are defined before using

Assume macros with the same prefix would be defined together.

include/ruby/io/buffer.h

@@ -56,13 +56,13 @@ enum rb_io_buffer_endian {
     RB_IO_BUFFER_LITTLE_ENDIAN = 4,
     RB_IO_BUFFER_BIG_ENDIAN = 8,
 
-#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#if defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
     RB_IO_BUFFER_HOST_ENDIAN = RB_IO_BUFFER_LITTLE_ENDIAN,
-#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#elif defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
     RB_IO_BUFFER_HOST_ENDIAN = RB_IO_BUFFER_BIG_ENDIAN,
-#elif REG_DWORD == REG_DWORD_LITTLE_ENDIAN
+#elif defined(REG_DWORD) && REG_DWORD == REG_DWORD_LITTLE_ENDIAN
     RB_IO_BUFFER_HOST_ENDIAN = RB_IO_BUFFER_LITTLE_ENDIAN,
-#elif REG_DWORD == REG_DWORD_BIG_ENDIAN
+#elif defined(REG_DWORD) && REG_DWORD == REG_DWORD_BIG_ENDIAN
     RB_IO_BUFFER_HOST_ENDIAN = RB_IO_BUFFER_BIG_ENDIAN,
 #endif