gst-plugins-bad: add option to build applemedia elements

Add an option to enable building of applemedia specific elements. I assume they are disabled by default as they may fail compilation on older SDK/toolchains(?). However they include some very useful OS X specific elements. They did compile and work fine for me for quite a while now. So I think it would be nice to expose these as an option.

Currently "applemedia" does include these elements:

    qtkitvideosrc: Video Source (QTKit)
    avfvideosrc: Video Source (AVFoundation)
    avfassetsrc: Source and decoder for AVFoundation assets
    atdec: AudioToolbox based audio decoder
    vtdec: Apple VideoToolbox decoder
    vtenc_h264: H.264 encoder