Unix absolute paths treated as relative when cross-compiling on Windows

This bug was found with Qt Creator extended with the 3rdparty plugin which enables cross-platform development for devices running Sailfish OS, a Unix-like OS. This means that even when running on a Windows host, project files may contain absolute Unix paths and those should be resolved against sysroot defined by the active Kit. The change https://codereview.qt-project.org/#/c/222357/ (qmake: require a drive in a DOS path for it to be absolute) breaks this, treating any absolute Unix path added to e.g. INCLUDEPATHS as a relative path and resolving it against the build directory instead of sysroot.