They are enabled if the hardware support the appropriated instructions.

Edit src/bench/bench_bitcoin.cpp and replace the line:

SHA256AutoDetect();

With:

std::cout << SHA256AutoDetect() <<std::endl;

Then

cmake -GNinja ..
ninja bitcoin-bench
./src/bench/bitcoin-bench -filter=SHA256

Check that the enabled features match your hardware capabilities by
looking at the CPU flags in /proc/cpuinfo.
Checked with all flags and without avx2.

Run the gitian builds.