CMake deprecation warnings

[Andrey Semashev]

When building libx265 3.1.1 on Ubuntu 19.04, CMake displays the following warnings:

-- cmake version 3.13.4
CMake Deprecation Warning at CMakeLists.txt:10 (cmake_policy):
The OLD behavior for policy CMP0025 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.

CMake Deprecation Warning at CMakeLists.txt:16 (cmake_policy):
The OLD behavior for policy CMP0054 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.

I believe, libx265 needs to be updated to avoid using deprecated policies.