Add 'dontoverwrite_' prefix for, e.g., configuration files?

[Simon Josefsson]

I have some configuration files:

confdir = $(CONFDIR)
conf_DATA = shishi.conf shisa.conf

During 'make install' they overwrite the currently installed copies,
if they are present.  Would it possible to do something like:

confdir = $(CONFDIR)
dontoverwrite_conf_DATA = shishi.conf shisa.conf

Which would make the Makefile snippet check if the file already exist,
and only install the file when it doesn't exist?

Other ideas on solving this problem would be appreciated, of course.

Thanks.