Ah, you didn't say you wanted to use the half baked Makefile system from within Studio! In that case go to the Project menu and select the bottom Configuration Options entry. In the dialg that appears click on the "Custom Options" picture/link at the left. Over in that right hand pane you'll see there's already an entry there that says "-std=gnu99" on the 3rd line. Just highlight this and click the [Edit] button.