This ensures at compile-time that Qt libraries do not use any APIs that
are not safe for use in application extensions, and fixes warning
messages that appear when linking to Qt libraries that are not built
with this flag, when used in an application extension.
This is especially important on watchOS where *all* "applications" are
actually application extensions, and on other Apple platforms if
application extensions are developed using Qt.
Task-number: QTBUG-40101
Change-Id: I022046f2584e0222253d33052b0abc221d7c93d6
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
944110089d