Taken from here:
https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/tools/documentation/gtk-doc/respect-xml-catalog-files-var.patch.

--- a/m4/gtkdoc_jh_check_xml_catalog.m4
+++ b/m4/gtkdoc_jh_check_xml_catalog.m4
@@ -5,8 +5,8 @@
 [
 	AC_REQUIRE([JH_PATH_XML_CATALOG],[JH_PATH_XML_CATALOG(,[:])])dnl
 	AC_MSG_CHECKING([for ifelse([$2],,[$1],[$2]) in XML catalog])
-	if $jh_found_xmlcatalog && \
-		AC_RUN_LOG([$XMLCATALOG --noout "$XML_CATALOG_FILE" "$1" >&2]); then
+	# empty argument forces libxml to use XML_CATALOG_FILES variable
+	if AC_RUN_LOG([$XMLCATALOG --noout "" "$1" >&2]); then
 		AC_MSG_RESULT([found])
 		ifelse([$3],,,[$3])
 	else
change='this.form.submit();'>
<option value='koszko' selected='selected'>koszko</option>
<option value='koszko-scripts'>koszko-scripts</option>
</select> <input type='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=4c0f0ab984f91756a01dc5f5191e3b5965247c95'>refs</a><a class='active' href='/guix/log/HACKING'>log</a><a href='/guix/tree/HACKING?id=4c0f0ab984f91756a01dc5f5191e3b5965247c95'>tree</a><a href='/guix/commit/HACKING?id=4c0f0ab984f91756a01dc5f5191e3b5965247c95'>commit</a><a href='/guix/diff/HACKING?id=4c0f0ab984f91756a01dc5f5191e3b5965247c95'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/HACKING'>
<input type='hidden' name='id' value='4c0f0ab984f91756a01dc5f5191e3b5965247c95'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=4c0f0ab984f91756a01dc5f5191e3b5965247c95'>root</a>/<a href='/guix/log/HACKING?id=4c0f0ab984f91756a01dc5f5191e3b5965247c95'>HACKING</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/HACKING?id=4c0f0ab984f91756a01dc5f5191e3b5965247c95&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-10-23 12:57:20 -0400'>2020-10-23</span></td><td><a href='/guix/commit/HACKING?id=73faee9fb25465f1b236f5f94e1052145655736f'>doc: Fix typo in HACKING.</a><span class='msg-avail'>...<span class='msg-tooltip'>* HACKING (Contributing): s/informations/information/.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2020-01-09 23:33:04 +0100'>2020-01-09</span></td><td><a href='/guix/commit/HACKING?id=2d315cd428484537e763d7f7c676dc5fff7995f6'>doc: Move "Commit Access" section from 'HACKING' to the manual.</a><span class='msg-avail'>...<span class='msg-tooltip'>* HACKING (Commit Access): Remove.
(Contributing): Update URL of the manual.
* doc/contributing.texi (Commit Access): New section.
(Submitting Patches): Add cross reference.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-01-09 23:33:04 +0100'>2020-01-09</span></td><td><a href='/guix/commit/HACKING?id=a7bde77d247ed0765f5378779d67b4668c5d7648'>doc: Add "Tracking Bugs and Patches" section.</a><span class='msg-avail'>...<span class='msg-tooltip'>* doc/contributing.texi (Tracking Bugs and Patches): New section.
(Submitting Patches): Refer to it.
* doc/guix.texi: Update copyright line.
* HACKING (Using emacs-debbugs): Remove.
</span></span></td><td>Ludovic Courtès</td></tr>