file(COPY_FILE): Add option to retry on Windows if input access fails

On Windows, a file may be inaccessible for a short time after it is
created.  This occurs for various reasons, including indexing, antivirus
tools, and NTFS's asynchronous semantics.  Add an `INPUT_MAY_BE_RECENT`
option to tell CMake that the input file may have been recently created
so that we can retry a few times to read it.