libvpx remove disable-runtime-cpu-detect

without runtime-cpu-detect, libvpx will use the compiler's supported optimizations, which may not exist on the cpu and cause a crash. confirmed this fix on #vp8 irc.
I personally experienced this trying to transcode video with ffmpeg to vp9 (this newer codec can use avx2 extensions, but they do not exist on my cpu [mid-2012 core i7])
also added head