a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowUnsupportedSystem = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowUnsupportedSystem = true; }
to ~/.config/nixpkgs/config.nix.


/nix/store/0a17pqjvqd4ys4rc9ifrndzqp8wnff7w-protobuf-3.6.1

copying path '/nix/store/3a9x9hklph2d2x36ydaq2l1gmm75if5z-SDL-1.2.15' from 'https://cache.nixos.org'...
copying path '/nix/store/jg1x98kz9ihcs6cwwvyr043kykwf4ba6-boost-1.55.0' from 'https://cache.nixos.org'...
copying path '/nix/store/b1zsd324dd839wiwyqcff8jpi4x2v0qm-SDL-1.2.15-dev' from 'https://cache.nixos.org'...
copying path '/nix/store/kxh0p8wcyrcqinvrpik8p3z9lgv8zskz-gsasl-1.8.0' from 'https://cache.nixos.org'...
copying path '/nix/store/286bm61w0vf2a0c88c703dnhpgxmlmri-smpeg-svn390' from 'https://cache.nixos.org'...
copying path '/nix/store/irjv8hv9hs6366mpkgfmcw2bpc1ppidp-tinyxml-2.6.2' from 'https://cache.nixos.org'...
copying path '/nix/store/8fic4mmm96bdfh6wvzmpyi4qf24d1hzq-SDL_mixer-1.2.12' from 'https://cache.nixos.org'...
copying path '/nix/store/gjj3kzya8fpnla4vmyys6b87cdjjd9lk-pokerth-1.1.1' from 'https://cache.nixos.org'...
/nix/store/gjj3kzya8fpnla4vmyys6b87cdjjd9lk-pokerth-1.1.1
/nix/store/p7qk7ndij6j18582sx0ad6vrs3z27hx5-protobuf-3.6.1

ln-UNIX stage/lib/libboost_locale.so
gcc.link.dll bin.v2/libs/serialization/build/gcc-7.3.0/release/threading-multi/libboost_wserialization.so.1.55.0
common.copy stage/lib/libboost_wserialization.so.1.55.0
ln-UNIX stage/lib/libboost_wserialization.so
...failed updating 3 targets...
...skipped 6 targets...
...updated 556 targets...
builder for '/nix/store/jbi31dss11irrbm3aln9q8q4dnwb8ha5-boost-1.55.0.drv' failed with exit code 1
cannot build derivation '/nix/store/4dpdgi62dxamwkghx2b44kn09z2c72ki-pokerth-1.1.1.drv': 1 dependencies couldn't be built
error: build of '/nix/store/4dpdgi62dxamwkghx2b44kn09z2c72ki-pokerth-1.1.1.drv' failed