Disable DirectFB automatic detection and mark it deprecated

[ChangeLog][Important Behavior Changes] Support for DirectFB is no
longer enabled by default, due to lack of development in upstream. To
re-enable the platform plugin, pass the -directfb option to configure. If
there is no interest in this platform, the support will be deprecated in
Qt 5.7 and will be removed in Qt 5.8.

See: http://lists.qt-project.org/pipermail/development/2016-March/025273.html


Change-Id: Icaa7fb2a490246bda156ffff143c62515a5f575b
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>