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

[Christoph Mathys]

> So it's just explicitly doing what is default :)
> If you want to override the default, you can set the mkspec explicitly in your Kit (Tools > Options > Build & Run > Kits)

Well, thank you both for the pointers! I had another look at our
buildscript, and I guess the default mkspec (which I didn't know
about) was changed in the Qt debian package between 4.6 and 4.8. So
suddenly, QtC and qmake (called without parameters) started to
disagree about the mkspec. Not wanting to harass every developer to
change their QtC configs, I fixed up the build script to be in sync
with QtC again.

Christoph