Define `MRB_ENDIAN_BIG` automatically; ref #4190

You had to define this macro on big endian platforms, but it is very
error-prone. So define the macro automatically if possible.

include/mrbconf.h

@@ -58,12 +58,16 @@
 # endif
 #endif
 
+/* define on big endian machines; used by MRB_NAN_BOXING, etc. */
+#ifndef MRB_ENDIAN_BIG
+# if defined(BYTE_ORDER) && defined(BIG_ENDIAN) && BYTE_ORDER == BIG_ENDIAN
+#  define MRB_ENDIAN_BIG
+# endif
+#endif
+
 /* represent mrb_value in boxed double; conflict with MRB_USE_FLOAT and MRB_WITHOUT_FLOAT */
 //#define MRB_NAN_BOXING
 
-/* define on big endian machines; used by MRB_NAN_BOXING */
-//#define MRB_ENDIAN_BIG
-
 /* represent mrb_value as a word (natural unit of data for the processor) */
 //#define MRB_WORD_BOXING