Refine compilation against shared FFmpeg.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@36976 b3059339-0415-0410-9bf9-f77b7e298cf2
Fix configure on a clean checkout.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@36977 b3059339-0415-0410-9bf9-f77b7e298cf2