Patch by Roman Tereshonkov and Kari Hautio for duplicity <= 0.6.25 to avoid a build failure with librsync
>= 1.0.0 (which is a security bugfix release). https://bugs.launchpad.net/duplicity/+bug/1416344 contains
the discussion and solution finding.

--- duplicity-0.6.25/duplicity/_librsyncmodule.c			2014-10-20 21:09:10.000000000 +0200
+++ duplicity-0.6.25/duplicity/_librsyncmodule.c.librsync-1.0.0		2015-03-02 01:29:00.000000000 +0100
@@ -67,8 +67,13 @@
   sm = PyObject_New(_librsync_SigMakerObject, &_librsync_SigMakerType);
   if (sm == NULL) return NULL;
 
+#ifdef RS_DEFAULT_STRONG_LEN
   sm->sig_job = rs_sig_begin((size_t)blocklen,
                              (size_t)RS_DEFAULT_STRONG_LEN);
+#else /* librsync >= 1.0.0 */
+  sm->sig_job = rs_sig_begin((size_t)blocklen,
+                             (size_t)8, RS_MD4_SIG_MAGIC);
+#endif
   return (PyObject*)sm;
 }