http://stackoverflow.com/a/25630208/2113226

all: observer

observer: main.o weather_center.o display.o subject.o observer.o
    g++ main.o weather_center.o display.o subject.o observer.o -o observer

main.o: main.cpp
    g++ -c main.cpp

weather_center.o: weather_center.cpp
    g++ -c weather_center.cpp

display.o: display.cpp
    g++ -c display.cpp

subject.o: subject.cpp
    g++ -c subject.cpp

observer.o: observer.cpp
    g++ -c observer.cpp

clean:
    rm -f  *o observer