Opt-out of the build on platforms where we won't build successfully

To allow the qt5.git integration, allow the build to succeed if the
repository is checked out on a platform that we don't maintain.

This is the initial list of platforms that we want the integration
to succeed and we can add new ones once we start maintaining them.

Change-Id: Idce2a84eb8d5b1ef7f14d399fe013f666b479108
Reviewed-by: Andras Becsi <andras.becsi@digia.com>