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/Makefile

@@ -140,8 +140,11 @@ ${PROG}: ${PROG}.c
 alt: data ${ALT_TARGET}
 	@${TRUE}
 
+# Variable to simplify reconfiguring P
+#
+P= '(A<<AP)'
 ${PROG}.alt: ${PROG}.alt.c
-	${CC} ${CFLAGS} $< -o $@ ${LDFLAGS}
+	${CC} ${CFLAGS} -DP=${P} $< -o $@ ${LDFLAGS}
 
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