This should prevent the avfoundation plugin from being built on system
that can not support using it (ex. 32bit builds on OS X 10.7)
Task-number: QTBUG-36239
Change-Id: I9f861b7580b0e015ddbc1231440d677bf6eee1ec
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>