Bug#939950: hkl: fails to build with gtk-doc 1.32

[Simon McVittie]

On Tue, 10 Sep 2019 at 14:49:05 +0200, Andreas Henriksson wrote:
> Your package seems to fail to build from source with the new gtk-doc-tools 1.32 (currently in experimental).
> The issue seems to be that your sgml files include a file that's no longer being generated when there's no content to put in it.

Library maintainers can either solve this class of bug by not
attempting to include the object tree at all, which for example is
appropriate if the library does not include any GObject subclasses:
https://gitlab.freedesktop.org/gstreamer/gst-libav/merge_requests/40

or by adding an empty <xi:fallback/> inside the <xi:include> element
for ../xml/tree_index.sgml (meaning "if you can't, substitute the empty
string instead") to return to the previous behaviour, which would look
something like this <xi:include> in GTK's documentation:

  <index id="api-index-3-24" role="3.24">
    <title>Index of new symbols in 3.24</title>
    <xi:include href="xml/api-index-3.24.xml"><xi:fallback /></xi:include>
  </index>

Regards,
    smcv