mklink: Do not needlessly overwrite linked files...

... on systems without symlinks.

Overwriting all the headers on each Configure causes full rebuild even if
nothing has changed.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1370)