Improve how to redefine sleep time in 2006/sloane

Now one can do

    make clobber SLEEP=555 alt

or whatever.

2006/sloane/Makefile

@@ -57,9 +57,12 @@ CSTD= -std=gnu99
 #
 ARCH=
 
+# variable to simplify redefining sleep duration for alt code
+#
+SLEEP= 75000
 # Defines that are needed to compile
 #
-CDEFINE= -DS=75000
+CDEFINE= -DS=${SLEEP}
 
 # Include files that are needed to compile
 #