Merge pull request #54408 from dtzWill/fix/smpeg-link-what-you-use

smpeg: fix build by linking against libX11 explicitly

pkgs/development/libraries/smpeg/default.nix

···
       --prefix PKG_CONFIG_PATH ":" "${SDL.dev}/lib/pkgconfig"
   '';
 
+  NIX_LDFLAGS = [ "-lX11" ];
+
   meta = {
     homepage = http://icculus.org/smpeg/;
     description = "MPEG decoding library";