make use of <module>_BUILD_PARTS

this avoids the nasty hack of running qmake in a submodule before its
dependencies have been built.

Change-Id: I75d2bdab7682e07bce2ef1a4807fb28672d8c4cf
Reviewed-by: Simo Fält <simo.falt@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>