In this http://doc.qt.nokia.com/latest/qmake-advanced-usage.html#adding-new-configuration-features documentation it says
With this addition to the CONFIG variable, qmake will search the locations listed above for the myfeatures.prf file after it has finished parsing your project file. On Unix systems, it will look for the following file:
3. myfeatures.prf (in the project's root directory)
Step 3: seems to be incomplete.
It should have been
3. myfeatures.prf (in the project's root directory) where .qmake.cache exitst (This is an empty file, which could be created)