Build developer documentation in a fix order

While working on reproducible builds for openSUSE, I found that when building the evolution package for openSUSE Tumbleweed, there were differences between each build in html files generated by gtk-doc.

Those are created in 5 dirs in 2 steps each.

The 1st step is gtkdoc-mkhtml and the 2nd step is gtkdoc-fixxref. And results there vary from racy parallelism (unless always building with -j1).

I tested that when dropping the fixxref part, results become reproducible.

I think, the 5 fixxref steps should only run after all 5 gtkdoc-mkhtml steps are done, which is currently not the case.

Example package-diff