Explicit control of enabled features

[Michael Orlitzky]

Now that the sage library has its own build system, we finally have
an opportunity to address the problems that arise from run-time
feature detection.

I've put three proposals up at,

https://github.com/sagemath/sage/discussions/41067

but may have failed to think of some good options.

The simplest of these (switch run-time to build-time) would change the
behavior of sage, particularly on binary distros, so I'm mentioning it
here to give everyone a chance to say "don't do that" to any options
they hate.