Boostbook problem with libxslt 1.27

Hi,

I upgraded libxslt on my computer, and it caused problems with
boostbook. I don't know if it's a bug in libxslt, or if we're doing
something we shouldn't (I think it's something I wrote, and I'm
clueless when it comes to xslt). I just checked in a simple fix, so
hopefully the build will work on trunk tonight. But don't upgrade
libxslt until you can use the fixed version.

Thanks,

Daniel