mg: fix cross-compilation

pkgs/applications/editors/mg/default.nix

@@ -11,6 +11,10 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-qnb0yB/NNJV257dsLmP84brajoRG03U+Ja1ACYbBvbE=";
   };
 
+  postPatch = lib.optionalString (stdenv.buildPlatform != stdenv.hostPlatform) ''
+    substituteInPlace configure --replace "./conftest" "echo"
+  '';
+
   enableParallelBuilding = true;
 
   makeFlags = [ "PKG_CONFIG=${buildPackages.pkg-config}/bin/${buildPackages.pkg-config.targetPrefix}pkg-config" ];