programs-info: add module and run install-info on system packages.

Fixes #25

modules/programs/info/default.nix

@@ -0,0 +1,35 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.programs.info;
+in
+
+{
+  options = {
+    programs.info.enable = mkOption {
+      type = types.bool;
+      default = true;
+      description = "Whether to enable info pages and the <command>info</command> command.";
+    };
+  };
+
+  config = mkIf cfg.enable {
+
+    environment.systemPackages = [ pkgs.texinfoInteractive ];
+
+    environment.pathsToLink = [ "/info" "/share/info" ];
+    environment.extraOutputsToInstall = [ "info" ];
+
+    environment.postBuild = ''
+      if test -w $out/share/info; then
+        shopt -s nullglob
+        for i in $out/share/info/*.info $out/share/info/*.info.gz; do
+          ${pkgs.texinfoInteractive}/bin/install-info $i $out/share/info/dir
+        done
+      fi
+    '';
+
+  };
+}