QtVersion: Mark system Qt as autodetectionSource "PATH"

... and use that to retrieve that version again.

This avoids trying to run qmake to make sure it is actually a qmake
and not just some binary with the same name in the PATH.

Change-Id: I7a2c3c85e81c77cc3b56dde088a73e065d03760c
Reviewed-by: Daniel Teske <daniel.teske@digia.com>