Especially older hardware doesn't support AVX instructions. DLib is
still functional there, but significantly slower[1].
By setting `avxInstructions` to false, DLib will be compiled without
this feature.
[1] http://dlib.net/compile.html
Initially the expression was quite small (just inherited properties from
`pkgs.dlib`), but the more it grows the better it is to store it into
its own file.