FWIW, this reminds me of that -mno-avx2 doesn't work here, so you'll need to do -U__AVX2__ in the CFLAGS, see for example:
https://github.com/kleisauke/wasm-vips/blob/9ff73c569c91ded6f8d8c7570967d0dadcf0134d/build.sh#L415

Anyway, that could probably be addressed in a follow-up.

FWIW, this reminds me of that -mno-avx2 doesn't work here, so you'll need to do -U__AVX2__ in the CFLAGS, see for example: https://github.com/kleisauke/wasm-vips/blob/9ff73c569c91ded6f8d8c7570967d0dadcf0134d/build.sh#L415

Anyway, that could probably be addressed in a follow-up.

Yes, that can be a separate non-NFC change

Even with the set, this does an awful lot of repeated work. But I guess performance doesn't really matter here?

The more efficient algorithm would be to create the set of user args, then linear search from highest to lowest feature in the feature tower to find the index of the highest enabled feature. Then you can iterate through the cflags up to that index.

I think this a nice simple cleanup regardless of possible future changes.