[Qt-creator] QtC 2.6 passes different -spec parameter than 2.5

[Christoph Mathys]

I installed QtCreator 2.6 today on my 64b Linux development machine,
previously I was using 2.5.1. We use qmake from the operating system,
which is 4.8.1.

The new qtcreator insists on calling my qmake with '-spec
linux-g++-64', where 2.5.1 called it with just '-spec linux-g++'. Why
is that? How can I change which spec parameter is passed by default?
What is wrong with linux-g++?

I wouldn't really want every developer to do some manual configuration
step after they upgraded to the new version of QtC.

Christoph