x86: Require an assembler able to cope with AVX instructions

All modern assemblers have this capability.  Older NASM versions
that lack the capability produce code that crashes at runtime,
so it's better to error out during the build process instead.

CC: libav-stable@libav.org

configure

@@ -3152,9 +3152,8 @@ EOF
             elf*) enabled debug && append YASMFLAGS $yasm_debug ;;
         esac
 
-        check_yasm "pextrd [eax], xmm0, 1" && enable yasm ||
+        check_yasm "vextractf128 xmm0, ymm0, 0" && enable yasm ||
             die "yasm not found, use --disable-yasm for a crippled build"
-        check_yasm "vextractf128 xmm0, ymm0, 0"      || disable avx_external
         check_yasm "vfmaddps ymm0, ymm1, ymm2, ymm3" || disable fma4_external
         check_yasm "CPU amdnop" && enable cpunop
     fi