MEXICO CITY — For the drug cartel boss who has everything, the latest piece of military hardware is a tank. Today’s competitive crime mafias in Mexico are no longer satisfied with bazookas or rocket-propelled grenades or land mines. The Mexican military has discovered that gangsters south of the Texas border are building armored assault vehicles, with gun turrets, inch-thick armor plates, firing ports and bullet-proof glass. More than 35,000 Mexicans have died in four years of drug violence, and in the border cities and farm towns of the northern state of Tamaulipus the fighting between the rival Gulf and Zeta cartels, and the military units that chase them, has been especially vicious.