cataclysm-dda: build recipe enhancements & gcc5 support

- Remove redundant platform check; meta.platforms is sufficient
- Use postPatch rather than override patchPhase entirely
- Strip -Werror
- Move build-time only dependencies to nativeBuildInputs

This also fixes gcc5 build, which fails due to a deprecated-declarations
warning (see https://hydra.nixos.org/build/33117020/nixlog/2/raw).