gmp{4,5}: Split into multiple outputs

Keep all versions consistent so that e.g. 'gmp.dev' is always available,
regardless of version.

pkgs/development/libraries/gmp/6.x.nix

@@ -10,7 +10,7 @@ let self = stdenv.mkDerivation rec {
     sha256 = "1mpzprdzkgfpdc1v2lf4dxlxps4x8bvmzvd8n1ri6gw9y9jrh458";
   };
 
-  #outputs TODO: split $cxx due to libstdc++ dependency; maybe port to gmp5;
+  #outputs TODO: split $cxx due to libstdc++ dependency
   # maybe let ghc use a version with *.so shared with rest of nixpkgs and *.a added
   # - see #5855 for related discussion
   outputs = [ "out" "dev" "info" ];