[docs] Rework the API reference version generation

Clutter copies the gtk-doc from the usual gtk+ template, and
has a version.xml.in containing only:

  @VERSION@

Without a newline at the end. Unfortunately, it appears that
autoconf has started adding a newline to the generated version.xml
which then is used as the payload for the "version" XML entity.

Instead of using a secondary file we can make configure generate
the whole clutter-docs.xml and cogl-docs.xml files from a template;
this way we also get the ability to substitute more autoconf variables
into the documentation -- if needs be.

doc/reference/cogl/Makefile.am

@@ -71,7 +71,6 @@ HTML_IMAGES=
 # Extra SGML files that are included by $(DOC_MAIN_SGML_FILE).
 # e.g. content_files=running.sgml building.sgml changes-2.0.sgml
 content_files = \
-	version.xml \
 	blend-strings.xml
 
 # SGML files where gtk-doc abbrevations (#GtkWidget) are expanded
@@ -94,4 +93,3 @@ include $(top_srcdir)/gtk-doc.make
 
 # Other files to distribute
 # e.g. EXTRA_DIST += version.xml.in
-EXTRA_DIST += version.xml.in

doc/reference/cogl/cogl-docs.xml.in

@@ -1,7 +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 version SYSTEM "version.xml">
+<!ENTITY version "@VERSION@">
 ]>
 <book id="index" xmlns:xi="http://www.w3.org/2003/XInclude">
 

doc/reference/cogl/version.xml.in

@@ -1 +0,0 @@
-@CLUTTER_VERSION@