GLOBAL MARKETS-Shares slide ahead of U.S. jobs data

* Europe shares drop on caution about U.S. jobs report * Euro steadies after hitting two-week low vs dollar, 3-month trough vs yen * Asian shares down for seventh straight week, worst run since 2011 * Oil sags back to $50 a barrel but up for second week By Marc Jones LONDON, Sept 4 (Reuters) - World shares slid towards their fourth weekly loss in the last five on Friday, as a boost from a supportive-sounding European Central Bank gave way to caution ahead of closely watched