build: allow use of system-installed brotli

brotli is available as a shared library since 2016, so it makes sense
to allow its use as a system-installed version.

Some of the infrastructure was in place already (node.gyp and
node.gypi), but some bits in the configure script here were missing.

Add them, keeping the default as before, to use the bundled version.

Refs: https://github.com/google/brotli/pull/421


Signed-off-by: André Draszik <git@andred.net>

PR-URL: https://github.com/nodejs/node/pull/32046


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>