Qt support: Fix detection of deployment target for Apple platforms
Qt support: Fix detection of deployment target for Apple platforms

In Qt 5.12, the deployment target information has moved from qmake.conf
into an included file, so let's expand the includes before looking for
the respective line.

Fixes: QBS-1434
Change-Id: I174c03cf227f319e33f484e82735d7ed550c2b11
Reviewed-by: Mitch Curtis <a74594a5101269175fe1b140badc4c0b6f699d25@qt.io>
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@qt.io>

qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs