Fix handling of LZ4 dependency version. (#21669)

* Fix handling of LZ4 dependency version. The strange split in how we handle our LZ4 dependency in the build system is an artefact of now removed features that depended on version 1.9 or later, combined with a lack of understanding of the evolution of the liblz4 API over the years. With the current state of things, none of our LZ4 dependent code actually gets built unless we find LZ4 version 1.9 or newer despite our checking for an older version afterwards, and we don’t actually depend on anything in the API that’s newer than version 1.7.1. Given this, just check for LZ4 version 1.7.1 or newer and don’t try any fallback checks. The only functional effect of this change aside from declaring our dependencies properly is that LZ4 support will be available on a small handful of systems it was not previously available on. * Correctly handle LZ4 as optional at configure time.