How to specify macx-llvm instead of macx-g++ as make spec on Mac OS 10.6.7

I use Qt Creator 2.0.1 on Mac OS 10.6.7 and wanted to try out the LLVM version of the g++ compiler, but I couldn't find out so far, how I can specify this deviation from the default:

Neither can I change macx-g++ to macx-llvm in Qt Creator's build steps settings, nor does it have any effect if I add the environment variable QMAKESPEC=macx-llvm to the build environment.

(The make spec macx-llvm really exists in Qt's mkspecs directory and can be specified successfully in a command line "qmake ... -spec macx-llvm").

Who can help?

Background: Apple claims that LLVM is twice as fast as the standard g++ compiler and uses LLVM in its new Xcode 4 IDE as default compiler.