file(RENAME): Add option to not replace existing path

Add a `NO_REPLACE` option that prevents overwriting `<newname>`
if it exists.

Help/command/file.rst

@@ -666,7 +666,8 @@ Examples of recursive globbing include::
 .. code-block:: cmake
 
   file(RENAME <oldname> <newname>
-       [RESULT <result>])
+       [RESULT <result>]
+       [NO_REPLACE])
 
 Move a file or directory within a filesystem from ``<oldname>`` to
 ``<newname>``, replacing the destination atomically.
@@ -677,6 +678,11 @@ The options are:
   Set ``<result>`` variable to ``0`` on success or an error message otherwise.
   If ``RESULT`` is not specified and the operation fails, an error is emitted.
 
+``NO_REPLACE``
+  If the ``<newname>`` path already exists, do not replace it.
+  If ``RESULT <result>`` is used, the result variable will be
+  set to ``NO_REPLACE``.  Otherwise, an error is emitted.
+
 .. _REMOVE:
 .. _REMOVE_RECURSE: