feat(third_party/lisp/s-xml): Check in sources & derivation

Checked in the sources for this because it is tracked upstream in CVS
and I can't be bothered to deal with that right now.

third_party/lisp/s-xml/default.nix

@@ -0,0 +1,17 @@
+# XML serialiser for Common Lisp.
+#
+# This system was imported from a Quicklisp tarball at 's-xml-20150608'.
+{ pkgs, ... }:
+
+pkgs.nix.buildLisp.library {
+  name = "s-xml";
+
+  srcs = [
+    ./src/package.lisp
+    ./src/xml.lisp
+    ./src/dom.lisp
+    ./src/lxml-dom.lisp
+    ./src/sxml-dom.lisp
+    ./src/xml-struct-dom.lisp
+  ];
+}