Merge commit '2b479bca'

These patches have not been merged at the request of Carl

* commit '2b479bca':
  build: Drop AVX assembly ifdefs
  x86: Require an assembler able to cope with AVX instructions

Conflicts:
	libavcodec/x86/fft.asm
Merged-by: NMichael Niedermayer <michaelni@gmx.at>