[Updated] Compile files in multiple folders

#MY NEW MAKEFILE
#YOU CAN ADD MUCH MORE THINGS
#THE "extra" IS ANOTHER FOLDER IN THE SAME PLACE HAS THIS MAKEFILE
BUILD_FILE = hello
CFLAGS=-c -Wall
LDFLAGS=

MAIN_OBJ = *.o
MAIN_CPP = *.cpp
EXTRA_OBJ = extra/*.o
EXTRA_CPP = extra/*.cpp

all: $(BUILD_FILE)

$(BUILD_FILE): $(MAIN_OBJ) $(EXTRA_OBJ)
    g++ $(LDFLAGS) *.o -o $(BUILD_FILE)

$(MAIN_OBJ): $(MAIN_CPP)
    g++ $(CFLAGS) $(MAIN_CPP)

$(EXTRA_OBJ): $(EXTRA_CPP)
    g++ $(CFLAGS) $(EXTRA_CPP)

clean:
    rm -rf *o $(BUILD_FILE)