Workaround for old dialog versions that print the version on stdout

* dialog.py: some dialog versions print the version on stdout when using
--print-version (e.g., 1.1-20100428). In this case, the output read on
stderr is empty. This commit works around this problem by rerunning the
dialog program, capturing its stdout instead of stderr. The penalty is
minimal since the backend version is cached.

dialog.py

@@ -1,7 +1,7 @@
 # dialog.py --- A Python interface to the ncurses-based "dialog" utility
 # -*- coding: utf-8 -*-
 #
-# Copyright (C) 2002, 2003, 2004, 2009, 2010, 2013, 2014  Florent Rougon
+# Copyright (C) 2002, 2003, 2004, 2009, 2010, 2013, 2014, 2015  Florent Rougon
 # Copyright (C) 2004  Peter Åstrand
 # Copyright (C) 2000  Robb Shecter, Sultanbek Tezadov
 #
@@ -1656,6 +1656,19 @@ def backend_version(self):
         """
         code, output = self._perform(["--print-version"],
                                      use_persistent_args=False)
+
+        # Workaround for old dialog versions
+        if code == self.OK and not (output.strip() or self.use_stdout):
+            # output.strip() is empty and self.use_stdout is False.
+            # This can happen with old dialog versions (1.1-20100428
+            # apparently does that). Try again, reading from stdout this
+            # time.
+            self.use_stdout = True
+            code, output = self._perform(["--stdout", "--print-version"],
+                                         use_persistent_args=False,
+                                         dash_escape="none")
+            self.use_stdout = False
+
         if code == self.OK:
             try:
                 mo = self._print_version_cre.match(output)