udunits: meta.platforms = lib.platforms.all

Relax the platform constrain,
since it seems not a platform-specific
and Homebrew have it in their package set

Specify meta.mainProgram

pkgs/development/libraries/udunits/default.nix

···
     '';
     license = licenses.asl20;
     maintainers = with maintainers; [ AndersonTorres pSub ];
-    platforms = platforms.linux;
+    platforms = platforms.all;
+    mainProgram = "udunits2";
   };
 }