nrf/Makefile: Use C11 instead of Gnu99.

Some constructs require C11 which GCC silently allows.

ports/nrf/Makefile

@@ -100,7 +100,7 @@ endif
 
 
 CFLAGS += $(CFLAGS_MCU_$(MCU_SERIES)) 
-CFLAGS += $(INC) -Wall -Werror -g -ansi -std=gnu99 -nostdlib $(COPT) $(NRF_DEFINES) $(CFLAGS_MOD)
+CFLAGS += $(INC) -Wall -Werror -g -ansi -std=c11 -nostdlib $(COPT) $(NRF_DEFINES) $(CFLAGS_MOD)
 CFLAGS += -fno-strict-aliasing
 CFLAGS += -Iboards/$(BOARD)
 CFLAGS += -DNRF5_HAL_H='<$(MCU_VARIANT)_hal.h>'