[thesuperbigfrog]

>> Can’t the OS / environment provide a “features_available” JSON blob listing all the features on the host system? Is AVX2 available on the cpu? OpenSSL? (And if so, where?) and what about kernel features like io_uring?

There are some utilities like pkg-config [1] and /proc/cpuinfo [2] that try to provide useful configuration information in distribution agnostic ways.

[1] https://en.wikipedia.org/wiki/Pkg-config

[2] https://www.baeldung.com/linux/proc-cpuinfo-flags

>> Doing haphazard feature detection by test compiling random hand written C programs in a giant sometimes autogenerated configure script is an icon of everything wrong with Unix.

True, but it works quite well which is why it is widely used. If you need to ensure that your C code will implement a desired feature, testing it with a small program before building makes a lot of sense. With different operating systems running various C compilers that all work slightly differently, it is a proven approach that achieves the needed outcome, however ugly it might be.