Creator adds -spec to qmake command line, overriding what is set in .qmake.cache

When using Qt Creator from the 2.4 branch to work on qt5/qtbase, it insists on adding '-spec xxx' to the qmake command line. However, the mkspec it chooses is not the one used to build Qt.

To reproduce, first build Qt with a different mkspec:
git clone qt/qtbase.git
cd qtbase
./configure -developer-buid -platform unsupported/macx-clang
make

Now, load qtbase.pro into Qt Creator, tell it to import the existing build. Once loaded, press the build button. The compile output window shows this:

Running build steps for project qtbase...
Starting: "/Volumes/Users/bhughes/src/qt5/qtbase/bin/qmake" /Volumes/Users/bhughes/src/qt5/qtbase/qtbase.pro -r -spec macx-g++42
...

This is the wrong mkspec.