docs: use xi:include to use the generated indexes if available.

This works transparantly for people with new enough gtk-doc (>=1.11) and
significantly improves the build time.

docs/reference/gio/gio-docs.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0"?>
 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
                "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [
+<!ENTITY % local.common.attrib "xmlns:xi  CDATA  #FIXED 'http://www.w3.org/2003/XInclude'">
 <!ENTITY version SYSTEM "version.xml">
 ]>
 <book lang="en" id="gio" xmlns:xi="http://www.w3.org/2003/XInclude">
@@ -142,22 +143,28 @@
       <xi:include href="xml/tree_index.sgml"/>
   </chapter>
   
-  <index>
+  <index id="api-index-full">
     <title id="index-all">Index</title>
+    <xi:include href="xml/api-index-full.xml"><xi:fallback /></xi:include>
   </index>
-  <index role="deprecated">
+  <index id="api-index-deprecated" role="deprecated">
     <title>Index of deprecated symbols</title>
+    <xi:include href="xml/api-index-deprecated.xml"><xi:fallback /></xi:include>
   </index>
-  <index role="2.18">
+  <index id="api-index-2-18" role="2.18">
     <title>Index of new symbols in 2.18</title>
+    <xi:include href="xml/api-index-2.18.xml"><xi:fallback /></xi:include>
   </index>
-  <index role="2.20">
+  <index id="api-index-2-20" role="2.20">
     <title>Index of new symbols in 2.20</title>
+    <xi:include href="xml/api-index-2.20.xml"><xi:fallback /></xi:include>
   </index>
-  <index role="2.22">
+  <index id="api-index-2-22" role="2.22">
     <title>Index of new symbols in 2.22</title>
+    <xi:include href="xml/api-index-2.22.xml"><xi:fallback /></xi:include>
   </index>
-  <index role="2.24">
+  <index id="api-index-2-24" role="2.24">
     <title>Index of new symbols in 2.24</title>
+    <xi:include href="xml/api-index-2.24.xml"><xi:fallback /></xi:include>
   </index>
 </book>