Whilst working on the Reproducible Builds effort [0] we noticed that
gtk-doc generates unreproducible output.

There will likely be more issues but this one is at least fairly
simple in that it iterates over a set structure when printing
some .devhelp2 headers, for example:

│ │ │ │ -    <sub name="Index of new API in 0.99.5" link="api-index-0-99-5.html"/>
│ │ │ │ -    <sub name="Index of new API in 0.9.22" link="api-index-0-9-22.html"/>
│ │ │ │      <sub name="Index of new API in 0.9.0" link="api-index-0-9-0.html"/>
│ │ │ │ -    <sub name="Index of new API in 0.9.1" link="api-index-0-9-1.html"/>
│ │ │ │ +    <sub name="Index of new API in 0.9.19" link="api-index-0-9-19.html"/>
│ │ │ │      <sub name="Index of new API in 0.99.8" link="api-index-0-99-8.html"/>
│ │ │ │ +    <sub name="Index of new API in 0.99.5" link="api-index-0-99-5.html"/>
│ │ │ │      <sub name="Index of new API in 1.0" link="api-index-1-0.html"/>
│ │ │ │ -    <sub name="Index of new API in 0.9.19" link="api-index-0-9-19.html"/>
│ │ │ │      <sub name="Index of new API in 0.9.2" link="api-index-0-9-2.html"/>
│ │ │ │ +    <sub name="Index of new API in 0.9.1" link="api-index-0-9-1.html"/>
│ │ │ │ +    <sub name="Index of new API in 0.9.22" link="api-index-0-9-22.html"/>

This was originally filed in Debian as #946311 [1].

 [0] https://reproducible-builds.org/
 [1] https://bugs.debian.org/946331



Signed-off-by: Chris Lamb <lamby@debian.org>