Building man pages fails if docbook.xsl is in a different location

[cjmayo]

Reported before at #105 and
https://bugs.launchpad.net/mosquitto/+bug/1269967

Can be made universal by using:

man/html.xsl

-	<xsl:import href="/usr/share/xml/docbook/stylesheet/docbook-xsl/html/docbook.xsl"/>
+	<xsl:import href="http:https://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl"/>

man/manpage.xsl

-	<xsl:import href="/usr/share/xml/docbook/stylesheet/docbook-xsl/manpages/docbook.xsl"/>
+	<xsl:import href="http:https://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl"/>

because xsltproc will use the system catalog to find the local path.

Can pass --nonet option to ensure the Internet is not used.

config.mk

-XSLTPROC=xsltproc
+XSLTPROC=xsltproc --nonet

[ralight]

A massive thanks for this, I know it has annoyed people for a long while. I've made the change on my local branch and it will be part of 1.5.6 soon.