Add option to link additional package outputs into system.path

This is necessary to get stuff like separate manpages, info files,
debug symbols, etc.

nixos/modules/config/system-path.nix

@@ -71,8 +71,16 @@ in
         # to work.
         default = [];
         example = ["/"];
-        description = "List of directories to be symlinked in `/run/current-system/sw'.";
+        description = "List of directories to be symlinked in <filename>/run/current-system/sw</filename>.";
       };
+
+      outputsToLink = mkOption {
+        type = types.listOf types.str;
+        default = [];
+        example = [ "doc" ];
+        description = "List of package outputs to be symlinked into <filename>/run/current-system/sw</filename>.";
+      };
+
     };
 
     system = {
@@ -119,7 +127,7 @@ in
     system.path = pkgs.buildEnv {
       name = "system-path";
       paths = config.environment.systemPackages;
-      inherit (config.environment) pathsToLink;
+      inherit (config.environment) pathsToLink outputsToLink;
       ignoreCollisions = true;
       # !!! Hacky, should modularise.
       postBuild =

pkgs/build-support/buildenv/default.nix

@@ -2,7 +2,7 @@
 # a fork of the buildEnv in the Nix distribution.  Most changes should
 # eventually be merged back into the Nix distribution.
 
-{ perl, runCommand }:
+{ perl, runCommand, lib }:
 
 { name
 
@@ -21,6 +21,10 @@
   # directories in the list is not symlinked.
   pathsToLink ? ["/"]
 
+, # The package outputs to include. By default, only the default
+  # output is included.
+  outputsToLink ? []
+
 , # Root the result in directory "$out${extraPrefix}", e.g. "/share".
   extraPrefix ? ""
 
@@ -36,7 +40,9 @@
 runCommand name
   { inherit manifest ignoreCollisions passthru pathsToLink extraPrefix postBuild buildInputs;
     pkgs = builtins.toJSON (map (drv: {
-      paths = [ drv ]; # FIXME: handle multiple outputs
+      paths =
+        [ drv ]
+        ++ lib.concatMap (outputName: lib.optional (drv.${outputName}.outPath or null != null) drv.${outputName}) outputsToLink;
       priority = drv.meta.priority or 5;
     }) paths);
     preferLocalBuild = true;

pkgs/top-level/all-packages.nix

@@ -261,7 +261,7 @@ let
     { substitutions = { inherit autoconf automake gettext libtool; }; }
     ../build-support/setup-hooks/autoreconf.sh;
 
-  buildEnv = callPackage ../build-support/buildenv {};
+  buildEnv = callPackage ../build-support/buildenv { }; # not actually a package
 
   buildFHSEnv = callPackage ../build-support/build-fhs-chrootenv/env.nix {
     nixpkgs      = pkgs;