doc-rst:c-domain: fix sphinx version incompatibility

The self.indexnode's tuple has changed in sphinx version 1.4, from a
former 4 element tuple to a 5 element tuple.

https://github.com/sphinx-doc/sphinx/commit/e6a5a3a92e938fcd75866b4227db9e0524d58f7cSigned-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sphinx/cdomain.py

@@ -29,11 +29,15 @@ u"""
 
 from docutils.parsers.rst import directives
 
+import sphinx
 from sphinx.domains.c import CObject as Base_CObject
 from sphinx.domains.c import CDomain as Base_CDomain
 
 __version__  = '1.0'
 
+# Get Sphinx version
+major, minor, patch = map(int, sphinx.__version__.split("."))
+
 def setup(app):
 
     app.override_domain(CDomain)
@@ -85,8 +89,14 @@ class CObject(Base_CObject):
 
         indextext = self.get_index_text(name)
         if indextext:
-            self.indexnode['entries'].append(('single', indextext,
-                                              targetname, '', None))
+            if major == 1 and minor < 4:
+                # indexnode's tuple changed in 1.4
+                # https://github.com/sphinx-doc/sphinx/commit/e6a5a3a92e938fcd75866b4227db9e0524d58f7c
+                self.indexnode['entries'].append(
+                    ('single', indextext, targetname, ''))
+            else:
+                self.indexnode['entries'].append(
+                    ('single', indextext, targetname, '', None))
 
 class CDomain(Base_CDomain):