Qt Creator re-runs qmake at every build

Every time I start a build, the qmake step re-runs.

My make file has:

# Command: /Users/Shared/qt/qt-4.8/bin/qmake -spec ../qt/mkspecs/unsupported/macx-clang CONFIG+=x86_64 -o Makefile ../2.6/qtcreator.pro

the qmake command line that Qt Creator runs is:

09:57:24: Starting: "/Users/Shared/qt/qt-4.8/bin/qmake" /Users/Shared/qt/2.6/qtcreator.pro -r -spec unsupported/macx-clang CONFIG+=x86_64

Setup with Qt build in /Users/Shared/qt/qt-4.8 and Qt Creator build in /Users/Shared/creator-2.6 (source in /Users/Shared/2.6)

Other strange thing is that my Kit doesn't have a mkspec set, and the default link points to macx-g++