weird
exit qt creator and try the following:
remove debug/release folders
remove the makefiles
remove any object_script files
remove xxx.pro.user file
try building again
If that doesn't work can you reproduce in a minimal project and attach it?

To the gurus: does anybody know how to show the environment info that gets shown when you do a configure? That's how I tracked down my messed up environment when I had all those compilers/SDKs installed.