Adding the sgml dtd for docbook 3.1

Required by iputils manpages.

pkgs/data/sgml+xml/schemas/sgml-dtd/docbook/3.1.nix

@@ -0,0 +1,33 @@
+{ stdenv, fetchurl, unzip }:
+
+let 
+
+  src = fetchurl {
+    url = http://www.oasis-open.org/docbook/sgml/3.1/docbk31.zip;
+    sha256 = "0f25ch7bywwhdxb1qa0hl28mgq1blqdap3rxzamm585rf4kis9i0";
+  };
+
+  isoents = fetchurl {
+    url = http://www.oasis-open.org/cover/ISOEnts.zip;
+    sha256 = "1clrkaqnvc1ja4lj8blr0rdlphngkcda3snm7b9jzvcn76d3br6w";
+  };
+
+in
+
+stdenv.mkDerivation {
+  name = "docbook-sgml-3.1";
+
+  unpackPhase = "true";
+
+  buildInputs = [ unzip ];
+
+  installPhase =
+    ''
+      o=$out/sgml/dtd/docbook-3.1
+      mkdir -p $o
+      cd $o
+      unzip ${src}
+      unzip ${isoents}
+      sed -e "s/iso-/ISO/" -e "s/.gml//" -i docbook.cat
+    '';
+}