Compilation: don't warn about using Sun proprietary APIs

E.g. we use Unsafe in quite a few places and this generates lots of
warnings, which we now suppress using the undocumented
-XDignore.symbol.file command-line option to javac.

Closes #6423

pom.xml

@@ -398,6 +398,9 @@
                          be fixed in version > 3.1
                      -->
                     <useIncrementalCompilation>false</useIncrementalCompilation>
+                    <compilerArgs>
+                        <arg>-XDignore.symbol.file</arg>
+                    </compilerArgs>
                 </configuration>
             </plugin>
             <plugin>