Fix installation of qml2puppet on Mac

The installation of the qml2puppet on Mac actually happens in the
build stage. However, we did use os.path.join() there to compute
the right installation folder, which was giving unexpected results due
to the ORIGINAL_QMAKE_QT_PRFXPATH being absolute.

Task-number: QTBUG-34606
Change-Id: I68cce88acbe7ff1e70a23a8ec5c16ba37bd76a92
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>