build: Allow to use external liblz4 instead of embedded copy

build: Allow to use external liblz4 instead of embedded copy

While we are likely will keep the embedded copy for a while to avoid a really
new dependency we allow now to use the external liblz4. You need at least
revision r120 and a package that ships the pc file for it.

Personally I would like to get rid of it rather sooner than later due to the
security implications and a bunch of code we ship but have no idea about.
Reality is that it will need some time until this new lib is actually
packaged and shipped with releases for a a majority of people.

This patch was co-worked with Doug Newgard <scimmia22@outlook.com>