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
