[REG 3.2-beta -> 3.2-rc1] OS X: Qt Creator doesn't use clang

QtCreator doesn't respect the compiler that's located in the current kit. In QtCreator 3.2-beta, this works, and in QtCreator 3.2-rc1 it doesn't work.

With 3.2-beta, clang++ is used which is correct. In QtCreator 3.2-rc1 using
the same kit and the same QBS project, QtCreator uses g++.