Improve how redefine sleep in 1992/kivinen

Using a Makefile variable one can do for instance

    make clobber SLEEP=30000 alt

to change the Z macro, the time to sleep.

1992/kivinen/Makefile

@@ -59,7 +59,7 @@ ARCH=
 
 # Defines that are needed to compile
 #
-CDEFINE= -DZ=20000
+CDEFINE= -DZ=${SLEEP}
 
 # Include files that are needed to compile
 #
@@ -138,6 +138,12 @@ ${PROG}: ${PROG}.c
 alt: data ${ALT_TARGET}
 	@${TRUE}
 
+# variable to control amount of time to sleep between writes for alt code.
+SLEEP= 20000
+${PROG}.alt: ${PROG}.alt.c
+	${CC} ${CFLAGS} -DZ=${SLEEP} $< -o $@ ${LDFLAGS}
+
+
 # data files
 #
 data: ${DATA}

1992/kivinen/README.md

@@ -39,9 +39,9 @@ Also try changing the speed this game moves. For instance if you wish to change
 the `usleep(3)` value to `30000` from `20000`, try:
 
 ```sh
-make clobber CDEFINE="-DZ=30000" alt # make it slower
+make clobber SLEEP=30000 alt # make it slower
 
-make clobber CDEFINE="-DZ=10000" alt # make it faster
+make clobber SLEEP=10000 alt # make it faster
 ```
 
 Then use the same syntax as above and described by the author.