ffmpeg port update r15943

[anddam (Andrea D'Amore)]

I updated the ffmpeg port to have alac encoding support in m4a files, I dropped the default variants as liba52 is not supported anymore by configure script, ffmpeg has its own library now. Fixing this ticket could probably close #17310 too.

I didn't crete patchfile, just change revision to 13, the svn revision variables to

set svn_rev 15943
set swscale_svn_rev 27970

and the libwscale in fetch phase to

system "svn co -r${swscale_svn_rev} svn://svn.mplayerhq.hu/mplayer/trunk/libswscale ${distpath}/${svn_rev}/trunk/libswscale"