cmake deprecation warning

[gregbenz]

CMake issues the following warning now:

CMake Deprecation Warning at CMakeLists.txt:22 (cmake_policy): The OLD behavior for policy CMP0091 will be removed from a future version of CMake.

The cmake-policies(7) manual explains that the OLD behaviors of all policies are deprecated and that a policy should be set to OLD only under specific short-term circumstances. Projects should be ported to the NEW behavior and not rely on setting a policy to OLD.