Respect user flags

--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 all:
-	gcc -o hexinject hexinject.c -lpcap
-	gcc -o prettypacket prettypacket.c
-	gcc -o hex2raw hex2raw.c
+	$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) -o hexinject hexinject.c -lpcap
+	$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) -o prettypacket prettypacket.c
+	$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) -o hex2raw hex2raw.c
 
 clean:
 	rm -f hexinject prettypacket hex2raw *~