Simplify `MRB_ENDIAN_BIG` macro definition; ref
`cpp` does not raise error on undefined macro access in condition.
MODIFY
include/mrbconf.h
include/mrbconf.h
/* 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
+# if BYTE_ORDER == BIG_ENDIAN
# define MRB_ENDIAN_BIG
# endif
#endif

Simplify `MRB_ENDIAN_BIG` macro definition; ref #4190

`cpp` does not raise error on undefined macro access in condition.