Change link flags order to fix linking error

packet-exchange/build/Makefile

@@ -25,20 +25,21 @@ CFLAGS += -MD -MP
 CFLAGS += -I $(SRCDIR)
 CFLAGS += -std=gnu99
 
-LDFLAGS = -pthread
+LLIBS = -pthread
 
 ifneq ($(WITH_XDP),)
 CFLAGS += -D WITH_XDP -I${HOME}/libbpf/include
-LDFLAGS += -L${HOME}/libbpf/src -l:libbpf.a
+LDIRS += -L${HOME}/libbpf/src
+LLIBS += -l:libbpf.a
 endif
 
 vpath %.c $(SRCDIR)
 
 $(SERVER_PROG): $(SERVER_OBJS)
-	$(CC) $(LDFLAGS) $^ -o $@
+	$(CC) $(LDFLAGS) $(LDIRS) $^ $(LLIBS) -o $@
 
 $(CLIENT_PROG): $(CLIENT_OBJS)
-	$(CC) $(LDFLAGS) $^ -o $@
+	$(CC) $(LDFLAGS) $(LDIRS) $^ $(LLIBS) -o $@
 
 -include $(subst .c,.d,$(SERVER_SRCS))
 -include $(subst .c,.d,$(CLIENT_SRCS))