It's the name that's NULL when an array isn't found

And missing prototypes

Untested

Not so much for old machines, as for getting decent performance
when not setting -march= (SSE2 is part of the amd64 ABI).

Should be able to handle all previous GRU variants and more.

Should work for both CBR and VBR. In the CBR case, we can make
CELT VBR and use DRED to fill the rest.

If configuring CELT for CBR but controlling the bitrate with
OPUS_SET_BITRATE rather than nbCompressedBytes, then the range
coder buffer would never get resized. AFAICT this could never
be triggered in Opus because CBR was also controlled by
nbCompressedBytes.