Bug 123786 Following bug 109980, the compliance 6.0 should be added to the compiler's compliance list.
If the JRE/JDK used is a 1.6/6.0 VM, then the compliance 6.0 should be available. Compliance 1.6 implies source level 1.6 and target level 1.6.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/BuildPathSupport.java
public static final String JRE_PREF_PAGE_ID= "org.eclipse.jdt.debug.ui.preferences.VMPreferencePage"; 
	
	private BuildPathSupport() {
		super();
	}
	
	/**
	 * Finds a source attachment for a new archive in the existing classpaths.
	 * @param elem The new classpath entry
	 * @return A path to be taken for the source attachment or <code>null</code>
	 */
	public static IPath guessSourceAttachment(CPListElement elem) {
		if (elem.getEntryKind() == IClasspathEntry.CPE_CONTAINER) {
			return null;
		}
		IJavaProject currProject= elem.getJavaProject(); 
		try {
			
			IJavaModel jmodel= JavaCore.create(ResourcesPlugin.getWorkspace().getRoot());
			IJavaProject[] jprojects= jmodel.getJavaProjects();
			for (int i= 0; i < jprojects.length; i++) {