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.

./gl_warp_sin.h
./gl_warp.c
./gl_texture.h
post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/qpijl8fwbvvgjn5bgpmwsma0mjd0rc6s-ezquake-3.0.1
shrinking /nix/store/qpijl8fwbvvgjn5bgpmwsma0mjd0rc6s-ezquake-3.0.1/bin/ezquake
strip is /nix/store/1hi76hr87bd1y1q1qjk0lv8nmcjip1c8-binutils-2.30/bin/strip
stripping (with command strip and flags -S) in /nix/store/qpijl8fwbvvgjn5bgpmwsma0mjd0rc6s-ezquake-3.0.1/bin
patching script interpreter paths in /nix/store/qpijl8fwbvvgjn5bgpmwsma0mjd0rc6s-ezquake-3.0.1
checking for references to /build in /nix/store/qpijl8fwbvvgjn5bgpmwsma0mjd0rc6s-ezquake-3.0.1...

                 from common.h:35,
                 from qwsvdef.h:26,
                 from pr_cmds.c:22:
/nix/store/vdl3qn9q5a6jjrb16dv56xgap7rms6ri-glibc-2.27-dev/include/bits/stdio2.h:64:10: note: '__builtin___snprintf_chk' output between 4 and 209 bytes into a destination of size 64
   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        __bos (__s), __fmt, __va_arg_pack ());
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
builder for '/nix/store/qbz6kyp42q1swbr3hn4w7555v41qwrkc-ezquake-3.0.1.drv' failed with exit code 2
error: build of '/nix/store/qbz6kyp42q1swbr3hn4w7555v41qwrkc-ezquake-3.0.1.drv' failed