failure building test-simd.C on darwin (no avx)

[collares]

This is with Clang 11. See #284 for an older version of this problem.

clang++ -DHAVE_CONFIG_H -I. -I..  -I..  -O2 -march=native -Wall -DNDEBUG -UDEBUG -I/nix/store/z69w4bjiqzdfkaqryvzqxqfw1aa31nbb-givaro-4.2.0/include    -g  -c -o test-simd.o test-simd.C
In file included from test-simd.C:30:
In file included from ../fflas-ffpack/fflas/fflas_simd.h:229:
In file included from ../fflas-ffpack/fflas/fflas_simd/simd128.inl:100:
../fflas-ffpack/fflas/fflas_simd/simd128_float.inl:242:37: error: '__builtin_ia32_vpermilps' needs target feature avx
        __m128d t1 = _mm_castps_pd (_mm_permute_ps (a, 0xd8));
                                    ^
/nix/store/9algqp49ny3ghkyvsnysh1k42ksac55p-clang-wrapper-11.1.0/resource-root/include/avxintrin.h:1088:11: note: expanded from macro '_mm_permute_ps'
  (__m128)__builtin_ia32_vpermilps((__v4sf)(__m128)(A), (int)(C))
          ^