Utilize build system better

Makefile

@@ -1,5 +1,21 @@
-all:
-	gcc -Wall -o coap endpoints.c main-posix.c coap.c -DDEBUG
+CFLAGS += -Wall -DDEBUG
+SRC = $(wildcard *.c)
+OBJ = $(SRC:%.c=%.o)
+DEPS = $(SRC:%.c=%.d)
+EXEC = coap
+
+all: $(EXEC)
+
+-include $(DEPS)
+
+$(EXEC): $(OBJ)
+	@$(CC) $(CFLAGS) -o $@ $^
+
+%.o: %.c %.d
+	@$(CC) -c $(CFLAGS) -o $@ $<
+
+%.d: %.c
+	@$(CC) -MM $(CFLAGS) $< > $@
 
 clean:
-	rm -f coap
+	@$(RM) $(EXEC) $(OBJ) $(DEPS)