bug#22131: gtk-doc fails to build on all platforms

[Mark H Weaver]

The gtk-doc builds started failing on all platforms when the following
security updates were added:

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* 3470fe0..: Mark H Weaver 2015-11-16 gnu: libsndfile: Add fixes for 
CVE-2014-9496 and CVE-2015-7805.
* b4a88dc..: Mark H Weaver 2015-11-15 gnu: libxslt: Add fix for CVE-2015-7995.
* ee6bb0c..: Mark H Weaver 2015-11-15 gnu: libxml2: Add fixes for 
CVE-2015-{1819,7941,7942,8035} and other bugs.
* 328a14e..: Mark H Weaver 2015-11-15 gnu: libpng: Update to 1.5.24 [fixes 
CVE-2015-8126].
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The failure occurs during 'configure', as follows:

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checking for XML catalog... 
/gnu/store/b0xwhwj1725c10xcylwd65hdr8avjfh8-docbook-xml-4.3/xml/dtd/docbook/catalog.xml
checking for xmlcatalog... 
/gnu/store/bhpr505s620iq099ndg8vp1ih4idv9fi-libxml2-2.9.2/bin/xmlcatalog
checking for DocBook XML DTD V4.3 in XML catalog... found
checking for DocBook XSL Stylesheets in XML catalog... not found
configure: error: could not find DocBook XSL Stylesheets in XML catalog
phase `configure' failed after 3.6 seconds
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Whereas in earlier successful builds, the corresponding output was:

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checking for XML catalog... 
/gnu/store/b0xwhwj1725c10xcylwd65hdr8avjfh8-docbook-xml-4.3/xml/dtd/docbook/catalog.xml
checking for xmlcatalog... 
/gnu/store/ny0sc2ix5w41z4dgc6nffhmpfy8izv99-libxml2-2.9.2/bin/xmlcatalog
checking for DocBook XML DTD V4.3 in XML catalog... found
checking for DocBook XSL Stylesheets in XML catalog... found
--8<---------------cut here---------------end--------------->8---

      Mark