Check what .cpp files you have in your directory. You may be linking in something you're not expecting when you're using wildcards. Better yet, use the power of make:
CXXFLAGS = -Wall -Wextra -O2
Game.exe: core_list.o main.o
$(CXX) $^ -o $@
clean:
rm Game.exe *.o *~
Now make knows that Game.exe depends on list.o and main.o, and knows how to create them without you adding specific rules. Better, if you modify list.cpp, make will only compile that source file, and then link using the previously compiled main.o, which for large projects can be a huge time saver. Even better, you could tell make that the .o files depend on list.h so they should be recompiled if you modify core_list.h
CXXFLAGS = -Wall -Wextra -O2
core_list.o: core_list.cpp list.h
main.o: main.cpp list.h
Game.exe: core_list.o main.o
$(CXX) $^ -o $@
clean:
rm Game.exe *.o *~