conflict of make autovaribable and shell register

[Robin]

hi,all:

I want to test a file exist in a makefile,
and write a below sentence:

EXISTED := $(shell test -e foo.c && echo $?)

but the shell function always return NULL,
i think make treated the shell register "$?" 
as her autovarible "$?".

how can i invoke the shell register "$?" into my makefile ?
how can I test some file is existed ?

best reguard
Robin.