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.