For more than a half century, the money sat largely untouched, accruing in a bank account managed by the Boston Symphony Orchestra. This summer, the BSO has tapped into the now $2.4 million Horblitt Fund. The cause: composer Elliott Carter, who turns 100 this year. Carter's work has been championed by BSO music director James Levine for years. Now, drawing as much as $400,000 from the fund, the BSO is pushing to spread word of Carter beyond the close-knit symphony world.