gendoc: fall back to pure modules if C extensions are not available (issue1711)

doc/gendoc.py

@@ -1,6 +1,8 b''
-import sys, textwrap
+import os, sys, textwrap
 # import from the live mercurial repo
 sys.path.insert(0, "..")
+# fall back to pure modules if required C extensions are not available
+sys.path.append(os.path.join('..', 'mercurial', 'pure'))
 from mercurial import demandimport; demandimport.enable()
 from mercurial.commands import table, globalopts
 from mercurial.i18n import gettext, _