gst1-libav: fix configure

Signed-off-by: Nicolas Thill <nico@openwrt.org>
gst1-libav: fix build on x86_64

Disable yasm explicitely to fix build on x86_64 when yasm is not installed

Signed-off-by: Nicolas Thill <nico@openwrt.org>
Merge pull request #343 from psycho-nico/my-fixes-20140922-01

gst1-libav fixes