BRUSSELS — Macedonia spun into political crisis Sunday as a referendum on a deal that would have unlocked the door to NATO and EU membership fell well short of the required turnout, raising the prospect that the Balkan nation would be blocked from those Western clubs for years to come. With most of the vote counted, only 37 percent of eligible Macedonians had answered the question of whether to accept an agreement to change the country’s name to North Macedonia in exchange for Greece’s lifting of its veto on NATO and European Union membership.