meta-makefile's targets don't depend on the respective sub-makefiles

I had a test project from Qt 4.8 which could be built with a very simple .pro file:
—
SOURCES = main.cpp
—

Now this doesn't build anymore, because include's like
#include <QApplication>
give a "No such file or directory" error. One has to add QT settings into the .pro file for every Qt module/component (sorry - what's the correct wording?) that will be used:
—
QT += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

SOURCES += main.cpp
—

Unfortunately, this still doesn't build. I first had to remove the build directory test-build-Desktop_Qt_5_0_1_MSVC2010_32bit-Debug

This is always reproducable. I can see that the "widgets" include path is not used at all until I manually clean or remove the build folder. Obviously this has to be done as soon as I would need, say, the network components.

PS: As you can see, I'm using the VS2010 version - don't know how it behaves with MinGW.