We use the keyword windows a lot in the project file, but I'm not sure if this is supported / correct. I was not able to find a reliable source that sheds some light on my question.
See: http://doc.qt.nokia.com/4.7-snapshot/qmake-advanced-usage.html
There is always win32 mentioned. Same for mac, which should perhaps be macx.
Anyway, I tried replacing all occurrences of windows with win32 and it compiles fine on my local setup. Perhaps a main dev can take a look at this.