Docbook 1.79.2 makes very few changes apart from changing the canonical URL
to cdn.docbook.org.  This patch adds support for the previous URL to avoid
breaking packages that still use that.

Adapted from Debian:
https://salsa.debian.org/debian/docbook-xsl/-/blob/master/debian/patches/0005-catalog.xml-Compatibility-with-1.79.1-or-earlier.patch

--- a/catalog.xml
+++ b/catalog.xml
@@ -5,4 +5,7 @@
   <rewriteSystem systemIdStartString="http://cdn.docbook.org/release/xsl/current/" rewritePrefix="./"/>
   <rewriteURI uriStartString="http://cdn.docbook.org/release/xsl/1.79.2/" rewritePrefix="./"/>
   <rewriteSystem systemIdStartString="http://cdn.docbook.org/release/xsl/1.79.2/" rewritePrefix="./"/>
+  <!-- Also support old URI of v1.79.1 or earlier -->
+  <rewriteURI uriStartString="http://docbook.sourceforge.net/release/xsl/current/" rewritePrefix="./"/>
+  <rewriteSystem systemIdStartString="http://docbook.sourceforge.net/release/xsl/current/" rewritePrefix="./"/>
 </catalog>
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<tr><td><span title='2020-03-21 20:17:20 +0100'>2020-03-21</span></td><td><a href='/guix/commit/gnu/packages/flex.scm?id=ee38d87c58aaf8f8c1b7599a598a56b94f09f711'>gnu: Remove flex@2.6.1.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/flex.scm (flex-2.6.1): Remove variable.
</span></span></td><td>Marius Bakke</td></tr>