Include: PoolAlloc: do not rely on CMake define

Include: PoolAlloc: do not rely on CMake define

On Windows, _DEBUG is defined by CMake if CMAKE_BUILD_TYPE is Debug. But
on other platforms, this is not the case and thus in debug mode, the
guard checks are not enabled.

Instead, rely on the NDEBUG define, which is always defined in release
mode (Release, RelWithDebInfo and MinSizeRel). This works reliably on
all platforms: It is also used to enable or disable assertions.