Warn users using media-libs/x264[10bit]

Using useflag 10bit for media-libs/x264 prevents simplescreenrecorder
from recording videos properly when x264 is used.
Warn stable users of media-libs/x264[10bit]
Warn for 10bit useflag for libav too
Merge pull request #6 from anders-larsson/10bit_useflag_warning

Issue #5: Warn users using media-libs/x264[10bit]