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.

thanks-for-help.md

@@ -3491,11 +3491,13 @@ pairs though that should be obvious) and also some that are correct including
 system("reset");` (via redefining `exit(3)` so that the column ending would be
 the same) so that as long as it runs to completion the terminal will be sane and
 the cursor will be visible. Using `atexit(3)` will not work if the program is
-killed and signals are ugly so this was not done.
+killed, signals are ugly and these would be messing with the entry too much so
+these were not done.
 
 Cody also added the alt version that lets one control how fast the painting is
 done, based on the author's recommendations, except that Cody made it
-configurable at compile time.
+configurable at compile time. The Makefile was modified in such a way as to make
+it very easy to redefine them at compile time.
 
 Cody also added the [try.sh](/2013/birken/try.sh) script for the entry and the
 [try.alt.sh](/2013/birken/try.alt.sh) script for the alt code.