Don't use _tzcnt instrinics with clang for windows w/o BMI.
Technically _tzcnt* intrinsics are only available when the BMI
instruction set is present. However the instruction encoding
degrades to "rep bsf" on older processors.
Clang for Windows debatably restricts the _tzcnt* instrinics behind
the __BMI__ architecture define, so check for its presence or
exclude the usage of these intrinics when clang is present.
See also:
https://ffmpeg.org/pipermail/ffmpeg-devel/2015-November/183404.html
https://bugs.llvm.org/show_bug.cgi?id=30506
http://lists.llvm.org/pipermail/cfe-dev/2016-October/051034.html
Signed-off-by: Dale Curtis <dalecurtis@chromium.org> Reviewed-by: Matt Oliver <protogonoi@gmail.com> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>