ccache looks at gcc command line and bases the cache on that so we
need to keep it static. See HOW IT WORKS in
http://ccache.samba.org/ccache-man.html

Index: README-builds.html
===================================================================
--- README-builds.html	(revision 237)
+++ README-builds.html	(working copy)
@@ -1367,7 +1367,10 @@
                 and the rest of the JDK will often be disk intensive.
                 <p>
                 Faster compiles are possible using a tool called
-                <a href="http://ccache.samba.org/">ccache</a>.
+				<a href="http://ccache.samba.org/">ccache</a>. To be effectively able to
+				use ccache set the BUILD_NUMBER environment variable. It by default has the
+				value of the current date which invalidates the ccache.
+				</p>
             </blockquote>
         </li>
         <li>