WASHINGTON — Secretary of State Hillary Rodham Clinton said Tuesday that the Arab world's revolutions could be remembered as "just a mirage in the desert" if leaders fail to make good on demands for greater democracy and economic opportunity.The warning from the Obama administration's top diplomat came amid increased signs of a backslide in the so-called Arab Spring, as Moammar Gadhafi persists with a bloody war against Libyan rebels and leaders from Yemen to Syria to Bahrain violently resist the calls from their people for a democratic transformation.