CMake: remove setting of policy CMP0043 to OLD.

CMake 3.0 ignores the suffixed COMPILE_DEFINITIONS_<CONFIG> variants if policy
CMP0043 is NEW, preferring instead to consume the unsuffixed COMPILE_DEFINITONS
property and evaluate generator expressions in it.

Setting the policy to OLD causes CMake 3.0 to not ignore those properties.

Use the suffixed properties for compatibility only for CMake versions older
than 3.0 using an explicit version check. Use the non-suffixed property with
generator expressions otherwise.

These definitions should mostly not be needed at all - CMake already sets
NDEBUG in suffixed CMAKE_CXX_FLAGS_<CONFIG> variables. The _DEBUG macro is
non-standard, but is used in several places in blender (usually in an OR
combination with the DEBUG macro). Additionally, blender overwrites (instead
of appending to) the CMAKE_CXX_FLAGS_<CONFIG> variables on WIN32.

Rather than try to fundamentally change how debug-macros are handled in
blender, change the buildsystem to keep the same behavior without requiring the
CMP0043 policy set to OLD.