bigint.h: simplified the condition.
#include <sys/types.h>
-#if !(defined(MRB_64BIT) || defined(MRB_INT64))
+#if defined(MRB_32BIT) && defined(MRB_INT32)
/*
* The values below are for 32 bit machines (i.e. machines with a
* 32 bit long type)
