RFE: provide PREFER_EXTERNAL_XXHASH configuration option

[keszybz]

In compilation for Fedora, I'm able to set -DPREFER_EXTERNAL_LZ4:BOOL=ON, -DPREFER_EXTERNAL_ZLIB:BOOL=ON, -DPREFER_EXTERNAL_ZSTD:BOOL=ON, but I see a bundled copy of xxhash is still being used. It'd be great to use the distro copy too.

[keszybz]

With the following:

rm -v plugins/codecs/ndlz/xxhash.?
sed -r -i '/xxhash.c/d' plugins/codecs/ndlz/CMakeLists.txt

everything builds fine and test_plugin_test_ndlz passes without issue.

So it seems that it'd be just a matter of skipping the xxhash.c from the sources and ensuring that xxhash.h from /usr/include is used.

[FrancescAlted]

Ok, but this will add the xxhash dependency, right? I suppose we would need something like PREFER_EXTERNAL_XXHASH. If you can provide a PR, I'll be happy to discuss more about this.