[CM] Backtrace in s7

[Bill Schottstaedt]

> Do you mean why they started to give
>these warnings in the first place? 

yes -- after googling around for awhile, I think this was introduced
in guile 2.0, and is not universally popular -- for example gnucash:

    ;; Turn off the scheme compiler's "possibly unbound variable" warnings. 
    ;; In guile 2.0 we get nearly 7500 of them loading the scheme files. 
    ;; This is the default value for auto-compilation-options without "unbound-variable". 
    (if (>= (string->number (major-version)) 2) 
        (set! %auto-compilation-options  
              '(#:warnings (arity-mismatch format duplicate-case-datum bad-case-datum)))) 

I still haven't found any rationale from the guile developers.  Anyway,
it's probably easy to add a switch.