This isn't a qmake issue, this is an issue of updates (or the lack thereof).
The "short" answer is:
The usage of double declaration was deemed pointless by the programming community, and thus support for such things was dropped from subsequent versions of many things. When you couple that with the fact that 15 Linux distros have 15 different views on what is "stable" and 2 1/2 years of no dev support, you're bound to have issues.

*also of note: if you look carefully, you'll see that the pull request you linked to was moot when it was initially posted, as the Bitcoin devs had fixed the issue the week before (over a year and a half ago). Again, making such issues the dev's fault and not that of Ubuntu (or any other distro), QT, boost, or anything else.