On MS-Win, QDir::absoluteFilePath() doesn't add a drive to a path that starts with a backslash

On MS-Win, a path such as \Users\qt\build\qtbase\bin\qmake.exe gets classified as "absolute" because it starts with a backslash, but its lack of a drive letter means it's not properly absolute.
QDir::absoluteFilePath() returns the path it's given if this is "absolute" even in this missing-drive sense.
This can result in problems for clients, e.g. qmake settings its QMAKE_QMAKE variable to the exact path given above, which leads to QNX builds on MinGW failing; this delayed the 5.6.0 release.