LIBREVILLE, GABON — Gabon’s transitional government said presidential elections would be held on April 12, a key step to reestablish civilian rule after a coup ended the Bongo dynasty’s decades-long reign. Government spokesman Seraphin Akure Davain made the announcement early Thursday following a Cabinet meeting. “Voting will start at 7 a.m. and end at 6 p.m. in line with current laws,” he said. The oil-rich central African country, which had been under the rule of the Bongo family for 55 years, adopted a new constitution in a November referendum. It provided for a maximum of two seven-year presidential terms, no