[Qt] Disable media support using QuickTime if deployment target != SDK

We link to a single libWebKitSystemInterface static lib, so unless the
deployment target matches the SDK let's not pretend like we'll be able
to run on anything lower than the SDK version.

Reviewed by Simon Hausmann.

* qmake/mkspecs/features/features.prf:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147540 268f45cc-cd09-0410-ab3c-d52691b4dbfc
[Qt] Update QMAKE_MAC_SDK check for Qt 5.1

Relies on QMAKE_MAC_SDK_VERSION set in features.prf

Reviewed by Simon Hausmann.

* WebCore.pri:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147541 268f45cc-cd09-0410-ab3c-d52691b4dbfc