Silence deprecation warning from CMake 3.0+

Silence deprecation warning from CMake 3.0+

Setting COMPILE_DEFINITIONS_<Config> has been deprecated since CMake 3.0.
However, the "proper" way of doing things now (generator expressions) have
been support only since CMake 2.8.10, and we require 2.8.9 currently.

Thus, we explicitly enable OLD behavior for CMP0043.