[REG Qt 4.8 -> 4.9] Cannot update translation file

1. Build Creator.
   If that matters, I built outside the source directory.
2. In the build directory's subdirectory share/qtcreator/translations, run make ts-de.
   It will fail with the message:

   lupdate error: Both project and source files / include paths specified.
   Makefile:555: recipe for target 'ts-de' failed
   make: *** [ts-de] Error 1
   

With Creator 4.8, this used to work.