When build the docs for debian package, it uses a sphinx building
helper, which does a sanity check for the output.
The sanity check complains missing data-url_root attribute in the
js tag. So this patch changes the js tag to use the same function like
the origin basic theme, which adds additional information like
data-url_root.
Ref: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=997437
With Sphinx 1.5, this is needed by searchtools.js to display the source
snippets (see sphinx-doc/sphinx#2454).
With earlier Sphinx versions, this is a no-op because the undefined variable
will evaluate to an empty string.
