CMakeLists.txt: Update for new subdirectory names
client: Update CMakeLists.txt

- Remove SFML as dependency
- Add parent directory to include search paths
- Specify which source files to build, as some source files are
  not in working condition.
nx: Include <cstddef> in lz4.hpp

size_t is defined in <cstddef>
GCC does not know what size_t is, unless it is included.