Improve how to redefine sleep in 1993/cmills

Using a Makefile variable one can now do

    make clobber SLEEP=200 alt

to change the time to sleep, rather than the more awkward redefining
CDEFINE.

1993/cmills/Makefile

@@ -57,7 +57,7 @@ ARCH=
 
 # Defines that are needed to compile
 #
-CDEFINE= -DZ=100
+CDEFINE= -DZ=${SLEEP}
 
 # Include files that are needed to compile
 #
@@ -141,8 +141,11 @@ chris: ${PROG}
 alt: data ${ALT_TARGET}
 	@${TRUE}
 
+# variable to control how long to sleep in alt version between updates
+SLEEP= 100
+
 ${PROG}.alt: ${PROG}.alt.c
-	${CC} ${CFLAGS} $< -o $@ ${LDFLAGS}
+	${CC} ${CFLAGS} -DZ=${SLEEP} $< -o $@ ${LDFLAGS}
 
 # data files
 #

1993/cmills/README.md

@@ -12,7 +12,7 @@ make alt
 To configure how many microseconds to sleep before updates try:
 
 ```sh
-make clobber CDEFINE="-DZ=200" alt
+make clobber SLEEP=200 alt
 ```
 
 NOTE: this entry requires `X11/Xlib.h` header file and the X11 library to
@@ -32,7 +32,6 @@ STATUS: known bug - please help us fix
 For more detailed information see [1993 cmills in bugs.md](/bugs.md#1993-cmills).
 
 
-
 ## To use:
 
 ```sh