Comment 5 for bug 1463273

It looks like the standard paths on Linux are broken in QT 4.
They where revamped in QT5
http://doc.qt.io/qt-5/qstandardpaths.html

Debian recommends using libqtxdg

Unfortunately it is not part of Ubuntu Precise.
Version 0.5 It is part of Ubuntu Trusty

The library supports Qt4 up to version 1.2
The it switches to the underlying Qt5 implementation form version 1.2
https://github.com/libqtxdg/libqtxdg/tree/da936792f2376327db2c287348738ede394e7bcc

I think the following approach is reasonable:
Use QString XdgDirs::dataHome(bool createDir)
from Trusty.
Hardcode "~/.local/share/Mixxx" for Precise
The xdg environment variables are not set anyway.