buildEnv: respect meta.outputsToInstall

As a result `systemPackages` now also respect it.
Only nix-env remains and that has a PR filed:
    https://github.com/NixOS/nix/pull/815

nixos/modules/config/system-path.nix

@@ -122,11 +122,7 @@ in
 
     system.path = pkgs.buildEnv {
       name = "system-path";
-      paths =
-        # The default output probably shouldn't be globally configurable.
-        # Services and users should specify them explicitly unless they want this default.
-        map (p: if p.outputUnspecified or false then p.bin or p.out or p else p)
-          config.environment.systemPackages;
+      paths = config.environment.systemPackages;
       inherit (config.environment) pathsToLink extraOutputsToLink;
       ignoreCollisions = true;
       # !!! Hacky, should modularise.

pkgs/build-support/buildenv/default.nix

@@ -48,7 +48,12 @@ runCommand name
             meta pathsToLink extraPrefix postBuild buildInputs;
     pkgs = builtins.toJSON (map (drv: {
       paths =
-        [ drv ]
+        # First add the usual output(s): respect if user has chosen explicitly,
+        # and otherwise use `meta.outputsToInstall` (guaranteed to exist by stdenv).
+        (if (drv.outputUnspecified or false)
+          then map (outName: drv.${outName}) drv.meta.outputsToInstall
+          else [ drv ])
+        # Add any extra outputs specified by the caller of `buildEnv`.
         ++ lib.filter (p: p!=null)
           (builtins.map (outName: drv.${outName} or null) extraOutputsToLink);
       priority = drv.meta.priority or 5;

pkgs/stdenv/generic/default.nix

@@ -225,6 +225,9 @@ let
             # If the packager hasn't specified `outputsToInstall`, choose a default,
             # namely `p.bin or p.out or p`;
             # if he has specified it, it will be overridden below in `// meta`.
+            #   Note: This default probably shouldn't be globally configurable.
+            #   Services and users should specify outputs explicitly,
+            #   unless they are comfortable with this default.
           // { outputsToInstall =
             let
               outs = outputs'; # the value passed to derivation primitive