Allow if in manifest variables.

The code supported this feature but the manifest DTD did now allow it.

doc/xml/dtd/manifest.dtd

@@ -3,6 +3,7 @@
 <!ELEMENT variable-list (variable+)>
 <!ELEMENT variable (#PCDATA)>
     <!ATTLIST variable key CDATA #REQUIRED>
+    <!ATTLIST variable if CDATA "default">
     <!ATTLIST variable eval CDATA "n">
 
 <!ELEMENT source-list (source+)>

doc/xml/release.xml

@@ -120,6 +120,10 @@
 
                         <p>Clarify that encryption is always performed client-side.</p>
                     </release-item>
+
+                    <release-item>
+                        <p>Allow <code>if</code> in manifest variables.</p>
+                    </release-item>
                 </release-improvement-list>
             </release-doc-list>