Improve how to redefine sleep time in 1993/plummer

Using a Makefile variable one may do

    make clobber SLEEP=50 alt

to change the Z macro to 50 or whatever they want.

1993/plummer/Makefile

@@ -58,7 +58,7 @@ ARCH=
 
 # Defines that are needed to compile
 #
-CDEFINE= -DZ=200
+CDEFINE= -DZ=${SLEEP}
 
 # Include files that are needed to compile
 #
@@ -138,8 +138,10 @@ ${PROG}: ${PROG}.c
 alt: data ${ALT_TARGET}
 	@${TRUE}
 
+# variable to control how long to sleep in alt version
+SLEEP= 200
 ${PROG}.alt: ${PROG}.alt.c
-	${CC} ${CFLAGS} $< -o $@ ${LDFLAGS}
+	${CC} ${CFLAGS} -DZ=${SLEEP} $< -o $@ ${LDFLAGS}
 
 # data files
 #

1993/plummer/README.md

@@ -44,7 +44,7 @@ make alt
 If you wish to change the time to sleep (default `200`) you can do so like:
 
 ```sh
-make clobber CDEFINE="-DZ=50" alt
+make clobber SLEEP=50 alt
 ```