Improve Makefile

- Use a bin/ directory for output
- Define `make all` as the default recipe

.gitignore

+bin/*

Makefile

-HEADERS = udp.h
 CFLAGS = -Wall -O3
 
+all: udp_send udp_recv
+
 udp_send: udp_send.c
-	gcc $(CFLAGS) -o $@ $?
+	gcc $(CFLAGS) -o bin/$@ $?
 
 udp_recv: udp_recv.c
-	gcc $(CFLAGS) -o $@ $?
-
-all: udp_send udp_recv
+	gcc $(CFLAGS) -o bin/$@ $?