Update release notes

Mention that --enable-gtk2-dependency is gone.

README.in

@@ -79,6 +79,9 @@ Release notes for 3.16
 * GTK+ now includes an OpenGL rendering widget. To support GL on various
   platforms, GTK+ uses libepoxy.
 
+* GTK+ no longer uses gtk-update-icon-cache during its build. The
+  --enable-gtk2-dependency configure option has been removed.
+
 Release notes for 3.14
 ======================
 

docs/reference/gtk/building.sgml

@@ -416,11 +416,6 @@ How to compile GTK+ itself
             <arg choice="plain">--enable-introspection=[no/auto/yes]</arg>
           </group>
           <sbr/>
-          <group>
-            <arg choice="plain">--enable-gtk2-dependency</arg>
-            <arg choice="plain">--disable-gtk2-dependency</arg>
-          </group>
-          <sbr/>
           <group>
             <arg choice="plain">--enable-installed-tests</arg>
             <arg choice="plain">--disable-installed-tests</arg>
@@ -637,19 +632,6 @@ How to compile GTK+ itself
         </para>
       </formalpara>
 
-      <formalpara>
-        <title><systemitem>--enable-gtk2-dependency</systemitem> or
-           <systemitem>--disable-gtk2-dependency</systemitem></title>
-
-        <para>
-          Whether to rely on an exiting gtk-update-icon-cache utility
-          instead of building our own. Distributions which are shipping
-          both GTK+ 2.x and GTK+ 3 may want to use this option to
-          avoid file conflicts between these packages.
-          The default is to build gtk-update-icon-cache.
-        </para>
-      </formalpara>
-
       <formalpara>
         <title><systemitem>--enable-installed-tests</systemitem> or
            <systemitem>--disable-installed-tests</systemitem></title>