Enable doc generation on non-windows platforms

This change enables running 'make docs' on platforms other than
Windows.

Change-Id: Ia6a03161db99fbe054f11e19f7cf152aa56574fb
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>