[developers] compiling latest revisions of PET (without XML support)

[Ben Waldron]

Stephan Oepen wrote:
> hei!
>
> call me old-fashioned, but i still compile my PET with `--disable-xml'
> (lacking all the right libraries on my laptop).  in recent revisions,
> that appears to no longer be possible.
>   
Sorry for that oversight. Fixed.
> ben, could i get you to wrap a few more #ifdef-s around your code? 
>
>                                        many thanks in advance  -  oe
>                                                    
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