Java version check (#9299)

build.properties

 javadoc.bottom=<i>Copyright &#169; 2000-2011 Vaadin Ltd. All Rights Reserved.</i>
 ivy.organisation=com.vaadin
 vaadin.vendor=Vaadin Ltd
-vaadin.java.version=6
+vaadin.java.version=1.6
 vaadin.version=0.0.0.noversion
 
 

build.xml

 		</subant>
 	</target>
 
-	<target name="buildorder">
+	<target name="buildorder" depends="official.build.checks">
 		<!-- Find out a good build order -->
 		<ivy:buildlist reference="build-path">
 			<fileset dir="." includes="**/build.xml">
 		</subant>
 	</target>
 
+	<target name="official.build.checks" if="official.build">
+		<condition property="java.version.matches">
+			<equals arg1="${ant.java.version}" arg2="${vaadin.java.version}" />
+		</condition>
+		<fail unless="java.version.matches" message="Java version is ${ant.java.version}, but Vaadin must be compiled with genuine Java ${vaadin.java.version} compiler." />
+		<echo>Java version is ${ant.java.version} as required.</echo>
+	</target>
+
 </project>