docbook autoconf check uses a different stylesheet than the one used to build the manpage

[edmonds]

the autoconf macro AX_CHECK_DOCBOOK_XSLT_MIN checks that the non-namespaced version of the DocBook stylesheets can be used:

http://docbook.sourceforge.net/release/xsl/current/...

while the Makefile actually uses the namespaced version:

http://docbook.sourceforge.net/release/xsl-ns/current/...

on debian systems the non-namespaced stylesheets are in the docbook-xsl package while the namespaced stylesheets are in the separate docbook-xsl-ns package, so it is possible for the configure check to succeed while the actual xsltproc invocation to build the manpage fails. the autoconf check needs to be updated in order to check for the namespaced stylesheets.