Fixes Makefile.config containing:
`HAVE_BROTLI = 1]`

Which was apparently harmless (unused, I think)
but is good to fix anyway.

allocate out-of-object to keep things neat.

Match what we do in other compression methods.
This is particularly important when using higher
compression levels, in order to ensure interrupts
are noticed in a somewhat responsive manner.

Breaking into chunks as hinted by zstd should help
this further, but this makes things much better already.

(not following those hints shouldn't be much worse
than what we have with other methods)