fix leap year calculation

sys/alpha/alpha/clock.c

@@ -63,7 +63,8 @@
 #define	SECDAY	((unsigned)(24*SECHOUR))	/* seconds per day */
 #define	SECYR	((unsigned)(365*SECDAY))	/* seconds per common year */
 
-#define	LEAPYEAR(year)	(((year) % 4) == 0)
+#define	LEAPYEAR(y)	\
+	(((y) % 4) == 0 && (((y) % 100) != 0 || ((y) % 400) == 0))
 
 device_t clockdev;
 int clockinitted;