QMakeStep: Honor declarative_debug configuration for imported builds

Honor the qmake line that is imported: If it contains declarative_debug
or QMLJSDEBUGGER_PATH, also set the option in the qmake step, if not
not.

Change-Id: Ibc524ff8d39b66f76cc79807264db0dd3534e2d7
Reviewed-on: http://codereview.qt-project.org/4090


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>