Add include flags to clang command

packet-exchange/build/Makefile

@@ -21,22 +21,26 @@ else
 CFLAGS = -Og -g -Wall -Wextra
 endif
 
+IFLAGS = -I $(SRCDIR)
+
 CFLAGS += -MD -MP
-CFLAGS += -I $(SRCDIR)
 CFLAGS += -std=gnu99
 
 LLIBS = -pthread
 
 ifneq ($(WITH_XDP),)
-CFLAGS += -D WITH_XDP -I${HOME}/libbpf/include
+CFLAGS += -D WITH_XDP
+IFLAGS += -I ${HOME}/libbpf/include
 LDIRS += -L${HOME}/libbpf/src
 LLIBS += -lelf -l:libbpf.a
 endif
 
+CFLAGS += $(IFLAGS)
+
 vpath %.c $(SRCDIR)
 
 xdp_kern.o: xdp_kern.c
-	clang -S -target bpf -D __BPF_TRACING__ -Wall -O2 -emit-llvm -c -g -o xdp_kern.ll $^
+	clang $(IFLAGS) -S -target bpf -D __BPF_TRACING__ -Wall -O2 -emit-llvm -c -g -o xdp_kern.ll $^
 	llc -march=bpf -filetype=obj -o $@ xdp_kern.ll
 
 ifneq ($(WITH_XDP),)