zstd: fix benchmarking

[mjg]

Summary

buildworld ends up defining the C standard as below C11, in turn zstd decides to use C90 timing primitives which by their own commentary give wrong results in multi-threaded setups.

This fixed speed reporting. See https://lists.freebsd.org/archives/freebsd-current/2021-December/001183.html for more details.

I don't care about the patch, feel free to drop it and fix in a different manner.

[mjg]

ping?

[diizzy]

We should probably also look into importing latest release since 1.5+ provides overall performance improvements.

[rew]

not sure if using CSTD=c11 in usr.bin/zstd/Makefile would be acceptable/work here, to avoid touching contrib code.

otherwise, looks good to me.

[cem]

Setting cstd seems strongly preferable vs making upstream updates more difficult.

[mjg]

• use CSTD=c11

I had no idea about the above var. I tried passing -std=c11 in cflags but that was failing.

[cem]

Thanks.