configure: update for "lavu/atomic: add support for the new memory model aware gcc built-ins"

This keeps the previous state to avoid breaking build on older gcc.
Better would be actual autodetection or using ffmpegs configure

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37315 b3059339-0415-0410-9bf9-f77b7e298cf2