Split compilation in smaller units

Makefile

@@ -1,7 +1,14 @@
-example: main.c foo.c
-	cc main.c foo.c -o example
+example: main.o foo.o
+	cc main.o foo.o -o example
+
+main.o: main.c
+	cc -c main.c -o main.o
+
+foo.o: foo.c
+	cc -c foo.c -o foo.o
+
 
 .PHONY: clean
 
 clean:
-	rm -f example
+	rm -f example *.o