emacs 下make

就简单的一个cpp文件 我在emacs下用shell命令g＋＋ 编译没有问题。但是想通过makefile编译一下问题就出来了。
makefile中
edit:a.cpp
g++ -o edit a.cpp也没有问题。
但是我分为2句话就出问题了
edit:main.o
g++ -o edit main.o
main.o:a.cpp
g++ -o main.o a.cpp
 
下面是报的错误。本人小白 求赐教！
本人环境 ubuntu 10。04 emacs 23
make -k
g++ -o main.o a.cpp
g++ -o edit main.o 
main.o: In function `_start':
(.text+0x0): multiple definition of `_start'
/usr/lib/gcc/i486-linux-gnu/4.4.3/../../../../lib/crt1.o:(.text+0x0): first defined here
main.o:(.rodata+0x0): multiple definition of `_fp_hw'
/usr/lib/gcc/i486-linux-gnu/4.4.3/../../../../lib/crt1.o:(.rodata+0x0): first defined here
main.o: In function `_fini':
(.fini+0x0): multiple definition of `_fini'
/usr/lib/gcc/i486-linux-gnu/4.4.3/../../../../lib/crti.o:(.fini+0x0): first defined here
main.o:(.rodata+0x4): multiple definition of `_IO_stdin_used'
/usr/lib/gcc/i486-linux-gnu/4.4.3/../../../../lib/crt1.o:(.rodata.cst4+0x0): first defined here
main.o: In function `__data_start':
(.data+0x0): multiple definition of `__data_start'
/usr/lib/gcc/i486-linux-gnu/4.4.3/../../../../lib/crt1.o:(.data+0x0): first defined here
main.o: In function `__data_start':
(.data+0x4): multiple definition of `__dso_handle'
/usr/lib/gcc/i486-linux-gnu/4.4.3/crtbegin.o:(.data+0x0): first defined here
main.o: In function `_init':
(.init+0x0): multiple definition of `_init'
/usr/lib/gcc/i486-linux-gnu/4.4.3/../../../../lib/crti.o:(.init+0x0): first defined here
/usr/lib/gcc/i486-linux-gnu/4.4.3/crtend.o:(.dtors+0x0): multiple definition of `__DTOR_END__'
main.o:(.dtors+0x4): first defined here
/usr/bin/ld: warning: Cannot create .eh_frame_hdr section, --eh-frame-hdr ignored.
/usr/bin/ld: error in main.o(.eh_frame); no .eh_frame_hdr table will be created.
collect2: ld returned 1 exit status
make: *** [edit] Error 1

把第一个g++换成ld看看。