Building against namespaced QT does not work on Windows

I configured Qt with "-qtnamespace QtCppIntegration" and Qt itselft build without problems. Building my own applications against that Qt ended in hundreds of linker errors.
My .qmake.cache contains the line "DEFINES = QT_NAMESPACE=QtCppIntegration". But "-DQT_NAMESPACE=QtCppIntegration" is not passed to g+ in the makefile of my own app.

Building that same app also with namespaced Qt works well on linux, btw.