Refactor mkFlag / shouldUsePkg into the nixpkgs libraries

pkgs/development/libraries/libssh/default.nix

@@ -7,9 +7,8 @@
 , openssl ? null, libgcrypt ? null
 }:
 
+with stdenv;
 let
-  shouldUsePkg = pkg: if pkg != null && stdenv.lib.any (x: x == stdenv.system) pkg.meta.platforms then pkg else null;
-
   # Prefer openssl
   cryptoStr = if shouldUsePkg openssl != null then "openssl"
     else if shouldUsePkg libgcrypt != null then "libgcrypt"