treewide: Make shouldUsePackages copypasta use meta.available

The old way depended on old list-of-strings `meta.platforms`, and was
not good for cross.

pkgs/development/libraries/wiredtiger/default.nix

@@ -15,7 +15,7 @@ let
   mkWith = mkFlag "with-" "without-";
   mkOther = mkFlag "" "" true;
 
-  shouldUsePkg = pkg: if pkg != null && any (x: x == stdenv.system) pkg.meta.platforms then pkg else null;
+  shouldUsePkg = pkg: if pkg != null && pkg.meta.available then pkg else null;
 
   optLz4 = shouldUsePkg lz4;
   optSnappy = shouldUsePkg snappy;