docs: Abort configure if docbook-xsl package is missing

The docbook-xsl package includes all the stylesheets required to build
the docs without internet access.

Test:
One way to emulate missing style sheets is to move /etc/xml/catalog file
to a different location. Doing so should cause configure to fail with
"checking for docbook stylesheets... no"

v2: add AC_MSG_RESULT (Pekka)

Signed-off-by: Harish Krupo <harishkrupo@gmail.com>