Use -std=c11

arduino/Makefile

@@ -13,7 +13,7 @@ OBJDUMP := avr-objdump
 AVRDUDE := avrdude
 
 OPTIM := -Os -g -fdata-sections -ffunction-sections -flto -fuse-linker-plugin -fipa-pta #-fgraphite-identity -floop-nest-optimize
-CFLAGS := $(OPTIM) -Wall -Wextra -std=c17 -DF_CPU=16000000L
+CFLAGS := $(OPTIM) -Wall -Wextra -std=c11 -DF_CPU=16000000L
 LDFLAGS := $(OPTIM) -Wl,--gc-sections -Wl,--print-gc-sections
 
 rwildcard = $(foreach d, $(wildcard $1*), $(filter $(subst *, %, $2), $d) $(call rwildcard, $d/, $2))

client/Makefile

@@ -3,7 +3,7 @@ name := GBCartRead
 dir_source := source
 dir_build := build
 
-CFLAGS := -O0 -g -Wall -Wextra -std=gnu17 $(CFLAGS)
+CFLAGS := -O0 -g -Wall -Wextra -std=gnu11 $(CFLAGS)
 
 CFLAGS += $(shell pkg-config --cflags libserialport)
 LDLIBS += $(shell pkg-config --libs libserialport)