Improve how to redefine sleep in 2013/birken alt

Now one can just do

    make clobber SLEEP=500 alt

or whatever.

2013/birken/Makefile

@@ -57,7 +57,7 @@ ARCH=
 
 # Defines that are needed to compile
 #
-CDEFINE= -DZ=15000
+CDEFINE=
 
 # Include files that are needed to compile
 #
@@ -139,13 +139,18 @@ all: data ${TARGET}
 ${PROG}: ${PROG}.c
 	${CC} ${CFLAGS} $< -o $@ ${LDFLAGS}
 
-${PROG}.alt: ${PROG}.alt.c
-	${CC} ${CFLAGS} $< -o $@ ${LDFLAGS}
 # alternative executable
 #
 alt: data ${ALT_TARGET}
 	@${TRUE}
 
+# Variable to control how to redefine sleep time in alt code
+#
+SLEEP= 15000
+
+${PROG}.alt: ${PROG}.alt.c
+	${CC} ${CFLAGS} -DZ=${SLEEP} $< -o $@ ${LDFLAGS}
+
 # data files
 #
 data: ${DATA}

2013/birken/README.md

@@ -40,7 +40,7 @@ compile time. If you wish to speed it up by 100% you can instead do:
 
 
 ```sh
-make clobber CDEFINE=-DZ=7500 alt
+make clobber SLEEP=7500 alt
 ```
 
 and then try `birken.alt` as above.