Improve redefining sleep time in 2000/thadgavin

Now one can do instead

    make SLEEP=555 SDL_SLEEP=555 alt

where SLEEP is the variable for the curses version and SDL_SLEEP is for
the SDL version. One need not specify both if they do not want to
redefine one or the other and of course one doesn't have to specify a
sleep time at all either.

2000/thadgavin/Makefile

@@ -56,9 +56,13 @@ CSTD= -std=gnu99
 #
 ARCH=
 
+# variables for redefining the time to sleep, primarily for alt code
+#
+SLEEP= 30
+SDL_SLEEP= 150000
 # Defines that are needed to compile
 #
-CDEFINE= -DZ=30 -DZS=150000
+CDEFINE= -DZ=${SLEEP} -DZS=${SDL_SLEEP}
 
 # Include files that are needed to compile
 #