Compile xdp kern with clang as a BPF target

packet-exchange/build/Makefile

@@ -6,7 +6,6 @@ SERVER_SRCS  = server.c
 SERVER_SRCS  += recv_packet.c
 SERVER_SRCS  += send_packet.c
 SERVER_SRCS  += utilities.c
-SERVER_SRCS  += xdp_kern.c
 
 CLIENT_SRCS  = client.c
 CLIENT_SRCS += recv_packet.c
@@ -36,8 +35,17 @@ endif
 
 vpath %.c $(SRCDIR)
 
+xdp_kern.o: 
+	clang -S -target bpf -D __BPF_TRACING__ -Wall -O2 -emit-llvm -c -g -o xdp_kern.ll xdp_kern.c
+	llc -march=bpf -filetype=obj -o xdp_kern.o xdp_kern.ll
+
+ifneq ($(WITH_XDP),)
 $(SERVER_PROG): $(SERVER_OBJS)
 	$(CC) $(LDFLAGS) $(LDIRS) $^ $(LLIBS) -o $@
+else
+$(SERVER_PROG): $(SERVER_OBJS), xdp_kern.o
+	$(CC) $(LDFLAGS) $(LDIRS) $^ $(LLIBS) -o $@
+endif
 
 $(CLIENT_PROG): $(CLIENT_OBJS)
 	$(CC) $(LDFLAGS) $(LDIRS) $^ $(LLIBS) -o $@