Improve Makefile for better binary compilation

User can change the compiler and the optimization level.
Also more generalized rules are used here.

Makefile

@@ -1,5 +1,18 @@
+CC?=gcc
+O_LVL?=3
 
-all:
-	gcc -O2 -o server server.c
-	gcc -O2 -o client client.c
+CFLAGS=-O$(O_LVL)
 
+BINARIES= \
+	server \
+	client
+
+all: $(BINARIES)
+
+$(BINARIES): %: %.c
+	$(CC) $(CFLAGS) $@.c -o $@
+
+clean:
+	rm $(BINARIES)
+
+.PHONY: all clean