how to tell gcc with c99 enable

just copy the make file here.

CC = gcc
CFLAGS = -Wall -std=c99 OUTFILE = outputfile OBJS = source.o SRCS = source.c $(OUTFILE): $(OBJS) $(CC) $(CFLAGS) -o $(OUTFILE) $(OBJS) $(OBJS): $(SRCS) $(CC) $(CFLAGS) -c $(SRCS)

The key point is -std=c99.