Refinements to macro facility

[Ben Pfaff]

I previously announced that I'd implemented the macro facility (the DEFINE
command) in PSPP.  I've spent the last few weeks working back and forth
with Frans Houweling (thank you Frans!) to make sure that it works properly
with the macros that he uses. I believe that it now passes all of his tests,
and I'm eager to hear from other users.