-CFLAGS := -Wall -Os -ffunction-sections -fdata-sections -mmcu=$(MCU) -DF_CPU=$(CLOCK) $(patsubst %,-D%,$(FEATURES)) -I../common
+CFLAGS := -Wall -std=c99 -Os -ffunction-sections -fdata-sections -mmcu=$(MCU) -DF_CPU=$(CLOCK) $(patsubst %,-D%,$(FEATURES)) -I../common