gnu: libxslt: Add zlib and libxml2 python module to inputs.

* gnu/packages/xml.scm (libxslt): Add input zlib and add libxml2 to PYTHONPATH.

gnu/packages/xml.scm

@@ -101,7 +101,19 @@ things the parser might find in the XML document (like start tags).")
     (synopsis "libxslt, a C library for applying XSLT stylesheets to XML documents")
     (inputs `(("libgcrypt" ,libgcrypt)
               ("libxml2" ,libxml2)
-              ("python" ,python)))
+              ("python" ,python)
+              ("zlib" ,zlib)))
+    (arguments
+      `(#:phases
+         (alist-replace
+          'configure
+          (lambda* (#:key inputs #:allow-other-keys #:rest args)
+            (let ((configure (assoc-ref %standard-phases 'configure))
+                  (libxml2 (assoc-ref inputs "libxml2")))
+              ;; FIXME: This should be done more centrally.
+              (setenv "PYTHONPATH" (string-append libxml2 "/lib/python2.7/site-packages"))
+              (apply configure args)))
+         %standard-phases)))
     (description
      "Libxslt is an XSLT C library developed for the GNOME project. It is
 based on libxml for XML parsing, tree manipulation and XPath support.")