When using AVIF_CODEC_AOM=SYSTEM, AVIF_CODEC_AOM_{DE,EN}CODE should be set to ON to make libavif link to AOM library binary. Otherwise it's compilable but won't be functional at all.
See also: https://github.com/AOMediaCodec/libavif/commit/f7ce882698307cad524603b9d528ce70c0d0d299