make install not overwriting progname.conf

[Lucas Nussbaum]

Hi,

I've been unable to find out how to achieve this behaviour :
make installs config file in a directory
if a config file already exists then
    write to progname.conf.new
    warn the user about possible changes
else
    write to progname.conf
fi

could someone point me to a program that has this behaviour, or tell be
how to acheive this behaviour ? (starting from scratch, please, because
I really can't figure out where to start)

I don't want the config files to the installed in sthing like share/,
and let the user copy them manually, because the program typically
installs in $HOME/progname (I've already managed to use automake for
that)

Thanks a lot,

lucas