Bug 53327 [Progress] Second manual build should optionally cancel the first
M7 If you select a Rebuild All from the workbench this is now run in the background. If it is run again then the user should be prompted to cancel either the new one or the one running. We should also have a "don't ask me again" option.
bundles/org.eclipse.ui.ide/extensions/org/eclipse/ui/actions/GlobalBuildAction.java
if (!verifyBuildersAvailable(roots))
		return;
		
	
	saveAllResources();
	
	doBuildOperation();
}
/**
 * Causes all editors to save any modified resources
 * depending on the user's preference.
 */
 void saveAllResources() {
	if (!BuildAction.isSaveAllSet())
		return;
		
	IWorkbenchWindow[] windows = PlatformUI.getWorkbench().getWorkbenchWindows();
	for (int i = 0; i < windows.length; i++) {
		IWorkbenchPage[] perspectives = windows[i].getPages();
		for (int j = 0; j < perspectives.length; j++)
			perspectives[j].saveAllEditors(false);
	}
}
/**
 * Checks that there is at least one project with a
 * builder registered on it.
 */
 boolean verifyBuildersAvailable(IProject[] roots) {
	try {
		for (int i = 0; i < roots.length; i++) {