Allow linking non-vendored lz4 and put vendored sources behind a feature

[figsoda]

If this is already possible, how do I do that?

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[folliehiyuki]

@picoHz I'm also interested in this. Can you look into the issue. Having an option to link to the system lz4 would be nice.

[picoHz]

It's a bit tricky because available features depend on the version of the installed liblz4. Even if you installed the latest version, some functions are not available in the dynamic-linked version.

[figsoda]

maybe we can do features for different versions