Bug 24199 New Classpath Variables
Why should a full rebuild of the workspace be required when you add a classpath variable? Because it's a new variable it doesn't affect any existing projects. It would be more convenient if a rebuild were not required when adding a new classpath variable. Thanks.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/VariableBlock.java
public void customButtonPressed(DialogField field, int index) {
			switch (index) {
			case 0: 
				editEntries(null);
				break;
			case 1: 
				List selected= fVariablesList.getSelectedElements();			
				editEntries((CPVariableElement)selected.get(0));
				break;
			}