Log Message:
Add docbook-xml and docbook-xsl to avoid nonet load failures as well as
add to xsltproc-nonet.mk a variable XSLTPROC_PATH allowing packages to
specify where to find locally files such as dtds, avoiding warnings like
'warning: failed to load external entity'.

At the same time add a BUILD_DEPENDS to libxslt for xsltproc-nonet.mk
and bump PKGREVISION.