Fix build with 5.8 and make embedded configurable

Avoid checking features only available in 5.9 directly. This also
changes the embedded-build configuration to being a feature that can be
set at configure time.

Change-Id: I9b42dcf696734a83e5c8d6e1cc07562cbeb3f993
Reviewed-by: Michael Brüning <michael.bruning@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>