CMP0057
-------

Disallow multiple ``MAIN_DEPENDENCY`` specifications for the same file.

CMake 3.3 and above no longer allow the same input file to be used
as a ``MAIN_DEPENDENCY`` in more than one custom command.

Listing the same input file more than once in this context has not been
supported by earlier versions either and would lead to build time issues
but was not diagnosed.

The ``OLD`` behavior for this policy is to allow using the same input file
in a ``MAIN_DEPENDENCY`` specfication more than once.
The ``NEW`` behavior is to disallow using the same input file in a
``MAIN_DEPENDENCY`` specification more than once.

This policy was introduced in CMake version 3.3.
CMake version |release| warns when the policy is not set and uses
``OLD`` behavior.  Use the :command:`cmake_policy` command to set
it to ``OLD`` or ``NEW`` explicitly.
