Tushar Maheshwari authored Dec 27, 2018 and Brad King committed Jan 16, 2019

This brings the functionality of `cmake -E create_symlink` and more to scripts.

The default behavior is to create hard links.
The `SYMBOLIC` argument can be used to create symlinks instead.

The `COPY_ON_ERROR` argument enables a fallback to copying the file in case the link fails.

The `RESULT <var>` retrieves the error message generated by the system.
It is set to "0" on success.

Fixes: #16926