configure: Treat warnings as errors in check_yasm()

yasm generates a warning instead of an error when "CPU amdnop" is not supported, so the check always succeeds.

HAVE_CPUNOP as true when the available yasm version doesn't support amdnop doesn't make sense.
Check http://fate.ffmpeg.org/log.cgi?time=20120901143315&log=compile&slot=x86_64-archlinux-gcc-oldyasm

 to see the amount of warnings it generates.

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