config.status links cp fallback

[Kevin Ryde]

A GMP user pointed out that the setups of as_ln_s include a fallback
to "cp" if "ln" fails, but that the same isn't true of config.status
when it sets up the AC_CONFIG_LINKS.

I don't have a set of actual circumstances where there's a problem,
though it would seem that if it's necessary to fall back to a copy in
one spot then very likely it is in both.

I guess AC_CONFIG_LINKS describes itself as creating links, but any
package depending on a link rather than a copy will already fail on a
system where links don't work, so I imagine creating a copy in that
case wouldn't make anything worse.