pharo-vm: stop trying to build on darwin

Building these pharo-vm sources on darwin may be possible but doesn't
make much sense because native darwin sources exist.

pkgs/development/pharo/vm/build-vm.nix

@@ -88,6 +88,10 @@ stdenv.mkDerivation rec {
     homepage = http://pharo.org;
     license = stdenv.lib.licenses.mit;
     maintainers = [ stdenv.lib.maintainers.DamienCassou ];
-    platforms = stdenv.lib.platforms.mesaPlatforms;
+    # Pharo VM sources are packaged separately for darwin (OS X)
+    platforms = with stdenv.lib;
+                  intersectLists
+                    platforms.mesaPlatforms
+                    (subtractLists platforms.darwin platforms.unix);
   };
 }