Bug 37353 add variable button can't add variable only edit. [build path}
Project->properties->java build path->Library tab->add variable button will only allow you to extend or edit a variable not add a variable. The only way to add a classpath variable is to go Window->preferences->java->classpath variables->new button.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/NewVariableEntryDialog.java
LayoutUtil.setHorizontalSpan(fVariablesList.getLabelControl(null), 2);
		
		GridData listData= (GridData) fVariablesList.getListControl(null).getLayoutData();
		listData.grabExcessHorizontalSpace= true;
		listData.heightHint= convertHeightInCharsToPixels(10);
		
		applyDialogFont(composite);		
		return composite;
	}
	
	public IPath[] getResult() {
		return fResultPaths;
	}
	
	/*
 	 * @see IDoubleClickListener#doubleClick(DoubleClickEvent)
 	 */
	private void doDoubleClick() {
		if (fIsValidSelection) {
			okPressed();
		} else if (fCanExtend) {
			extendButtonPressed();
		}
	}
	
	private void doSelectionChanged() {
		boolean isValidSelection= true;
		boolean canExtend= false;
		StatusInfo status= new StatusInfo();