Using the USE_AUTO_VECTOR option.

[Gliese852]

I need to run dlib on a machine without avx and even sse4, but the build, including python module, is done on a machine that has these extensions. While solving the problem, I came across this option, but I had two one question:

1. Based on this line, should I set the USE_AUTO_VECTOR to true to disable using vector extensions? Shouldn't it be the opposite?

2. When building the pybind11 module, this option has no effect, is this intentional?

[davisking]

Pass in --no USE_AVX_INSTRUCTIONS when building the python package and it will not use avx instructions.