我在makefile中遇到以下错误：

/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crt1.o：在函数 _start': (.text+0x20): undefined reference to main ' collect2: error: ld returned 1 exit status makefile:7: recipe for target ' runme'失败make：*** [runme]错误1

这是我的makefile代码：

CXXFLAGS+=-std=c++11
OBJ=main.o Shape.o
all: runme
runme: main.o Shape.o
    g++ $(OBJ) $(CXXFLAGS) -o runme
main.o: main.cpp Shape.hpp
    g++ $(CXXFLAGS) main.cpp -o main.o
Shape.o: Shape.hpp Shape.cpp
    g++ $(CXXFLAGS) Shape.cpp -o Shape.o
clean: 
    rm -f *.o
distclean: clean
    rm -f runme
c++ makefile