Improve alt build of 1989/ovdluhe

One can now do

    make clobber P=5 alt

instead of the more awkward

    make clobber CDEFINE="-DP=5" alt

The try.alt.sh script was added and allows one to do

    P=5 ./try.alt.sh

If P is not specified it uses the default of the alt code (which checks
if it's defined first via #ifdef..#endif). Otherwise it uses the new
value. Of course if one specifies not an int or too high value (or maybe
too low - didn't try that) the program will fail. That is the fault of
the user. The author does note a reasonable range to try.

1989/ovdluhe/README.md

@@ -35,11 +35,12 @@ it's a `#define` it's easy to set up.
 ### Alternate build:
 
 ```sh
-make CFLAGS="-DP=9" clobber alt
+make P=9 clobber alt
 ```
 
 You do not have to define `P` but if you do you can replace the value with
-whatever you wish, as long as it's an int.
+whatever you wish, as long as it's an int (or evaluates to an int like the
+default `(A<<AP)` does).
 
 
 ### Alternate use:
@@ -48,6 +49,16 @@ whatever you wish, as long as it's an int.
 ./ovdluhe.alt
 ```
 
+### Alternate try:
+
+```sh
+./try.alt.sh
+
+P=5 ./try.alt.sh
+```
+
+NOTE: if you specify too high a value for `P` the program might fail.
+
 
 ## Judges' remarks: