Book's Makefile leaves temporary files behind

[Gerard Beekmans]

Hiya,

I can't remember if there was an ulterior motive for this or not. After 
generating the book, a few temporary files were left behind:

- lfs-full.xml
- lfs-html.xml
- lfs-pdf.fo
- lfs-pdf.xml

These files are all removed by the 'tmpdir' target that runs just before 
'validxml' so it's not a big deal. Except it's clutter *after* 
generation unless there's a reason to keep those files around? Besides 
for debugging purposes that is. For that we could easily introduce a 
variable to switch on/off that behaviour.

Comments?

Gerard