Improve how to set number of arrows 1994/dodsond2

Using a Makefile variable one can now do

    make clobber ARROWS=5 alt

or whatever they wish to set it to. The code already checked the range.

1994/dodsond2/README.md

@@ -46,7 +46,7 @@ The default number of arrows in this version is 3 but if you want to change it,
 say to 5, try:
 
 ```sh
-make clobber CDEFINE="-DW=5" alt
+make clobber ARROWS=5 alt
 ```