docs: autodetect docbook-xsl path and document dependencies

Auto-detect DocBook XSL stylesheet locations in the doc makefile so the
build works across distro-specific install paths without manual symlinks.

Document required Debian/Ubuntu packages in doc/README and refresh
copyright year text in the XML guidelines document.

doc/README

@@ -1,3 +1,12 @@
 The developer documentation here is generated from the DocBook format.
 
+On Debian/Ubuntu, install these packages to build the guidelines:
+
+  make
+  xsltproc
+  docbook-xsl
+  docbook-xml
+
+The makefile auto-detects the DocBook XSL stylesheet path used by
+different distributions.
 

doc/developer-guidelines.xml

@@ -13,7 +13,7 @@
     <pubdate>2013</pubdate>
     <title>Monitoring Plugins Development Guidelines</title>
     <copyright>
-      <year>2000 - 2024</year>
+      <year>2000 - 2026</year>
       <holder>Monitoring Plugins Development Team</holder>
     </copyright>
   </articleinfo>
@@ -22,7 +22,7 @@
     <para>The purpose of these guidelines is to provide a reference for
     the plugin developers and encourage the standardization of the
     different kinds of plugins: C, shell, perl, python, etc.</para>
-    <para>Monitoring Plugins Development Guidelines Copyright (C) 2000-2024
+    <para>Monitoring Plugins Development Guidelines Copyright (C) 2000-2026
         (Monitoring Plugins Team)</para>
     <para>Permission is granted to make and distribute verbatim
         copies of this manual provided the copyright notice and this

doc/makefile

@@ -1,7 +1,19 @@
 # Quick makefile to create developer-guidelines.html
 
+DOCBOOK_XSL := $(firstword $(wildcard \
+	/usr/share/sgml/docbook/xsl-stylesheets/html/docbook.xsl \
+	/usr/share/xml/docbook/stylesheet/docbook-xsl/html/docbook.xsl))
+
+ifeq ($(DOCBOOK_XSL),)
+$(error Could not find DocBook XSL stylesheet; install docbook-xsl)
+endif
+
 developer-guidelines.html: developer-guidelines.xml developer-guidelines.css
-	xsltproc --nonet --stringparam html.stylesheet developer-guidelines.css -o developer-guidelines.html /usr/share/sgml/docbook/xsl-stylesheets/html/docbook.xsl developer-guidelines.xml
+	xsltproc --nonet \
+		--stringparam html.stylesheet developer-guidelines.css \
+		-o developer-guidelines.html \
+		$(DOCBOOK_XSL) \
+		developer-guidelines.xml
 
 clean:
 	rm -f developer-guidelines.html developer-guidelines.html.last jade-out.fot