Improve how to redefine sleep in 2013/birken alt

Now one can just do

    make clobber SLEEP=500 alt

or whatever.

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.