define Rand() in isaac4 config

Rand() was typically defined to random() or to rand().

gcc seems to provide a random() to link to on linux
when sys/share/random.c is linked in, but other platforms
such as Windows got an undefined refence to random()
when RANDOM wasn't defined.

The only direct use seems to be in get_rnd_txt() these
days, in rumors.c

Under the USE_ISAAC64 config, neither srandom()
nor srand() are being invoked to seed those routines,
and it really should be using isaac64 when USE_ISAAC64
is defined anyway.

include/pcconf.h

@@ -236,12 +236,16 @@
 #include <time.h>
 #endif
 
+/* the high quality random number routines */
+#ifdef USE_ISAAC64
+#define Rand() rn2(RAND_MAX)
+#else
 #ifdef RANDOM
-/* Use the high quality random number routines. */
 #define Rand() random()
 #else
 #define Rand() rand()
 #endif
+#endif
 
 #ifndef TOS
 #define FCMASK 0660 /* file creation mask */