Fix sphinx 'manpage' role

Building the documentation with manpages fails with sphinx >= 1.6
because of an internal change [1]. By explicitly setting the domain to
'None' when translating the 'manpage' node into a 'doc' node, sphinx's own
transformation to handle this change is triggered.

[1] sphinx-doc/sphinx#3698

lib/build/sphinx_ext.py

@@ -360,6 +360,7 @@ def _ManPageNodeClass(*args, **kwargs):
 
   # Force custom title
   kwargs["refexplicit"] = True
+  kwargs["refdomain"] = None
 
   return sphinx.addnodes.pending_xref(*args, **kwargs)