CMP0013
-------

Duplicate binary directories are not allowed.

CMake 2.6.3 and below silently permitted add_subdirectory() calls to
create the same binary directory multiple times.  During build system
generation files would be written and then overwritten in the build
tree and could lead to strange behavior.  CMake 2.6.4 and above
explicitly detect duplicate binary directories.  CMake 2.6.4 always
considers this case an error.  In CMake 2.8.0 and above this policy
determines whether or not the case is an error.  The OLD behavior for
this policy is to allow duplicate binary directories.  The NEW
behavior for this policy is to disallow duplicate binary directories
with an error.

This policy was introduced in CMake version 2.8.0.  CMake version
|release| warns when the policy is not set and uses OLD behavior.  Use
the cmake_policy command to set it to OLD or NEW explicitly.
