mklink: Do not needlessly overwrite linked files...

... on systems with symlinks.

Creating or overwriting a symlink sets the file ctime to the current time.
This causes needless rebuilds because the time of all the headers is
changed, and apparently make considers the link's time rather than the
time of the target.

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