don't needlessly rebuild the compression libraries

[cldellow]

Arrow will download and build the compression libraries (lz4, snappy, zlib, zstd, brotli).

And then we'll do it, too, because we want to compile the static libs with -fPIC.

I think we can opt out of this by passing the right flag to arrow's cmake and setting the appropriate flags in https://github.com/apache/arrow/blob/master/cpp/thirdparty/README.md

[xhochy]

Arrow also builds them with -fPIC so you could also use its builds. Otherwise setting the *_HOME variables will tell Arrow to pick up your builds.

[cldellow]

Ah, thank you! That's much preferable.

PS - thank you and all the other arrow/parquet committers for providing these libraries, they're fantastic.