Fix self-hosted build error on macOS with Qt 5.7
Fix self-hosted build error on macOS with Qt 5.7

This was caused by a change in Qt 5.7 which #includes the Availability.h
header which is not in moc's default search path. Qbs 1.6 contains a
patch to supply moc with the correct header search paths, therefore this
workaround is required until Qbs requires version 1.6 of itself for self
hosted builds.

Change-Id: I38db36235ac34a06a1a8e33fb474bb72b2928184
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@qt.io>

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