Make console example work with my GCC version (add _GNU_SOURCE)

Author: maximevince

examples/stm32/f0/stm32f0-discovery/usart_stdio/usart_stdio.c

@@ -18,6 +18,7 @@
  * along with this library.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#define _GNU_SOURCE         /* See feature_test_macros(7) */
 #include <libopencm3/stm32/rcc.h>
 #include <libopencm3/stm32/gpio.h>
 #include <libopencm3/stm32/usart.h>

examples/stm32/f4/stm32f429i-discovery/lcd-dma/console.c

@@ -22,6 +22,7 @@
  *
  */
 
+#define _GNU_SOURCE
 #include <stdint.h>
 #include <stdio.h>
 #include <setjmp.h>