Snappy is not enabled by default in cmake build, enable it

[Magister]

For some reason librdkafka's cmake build does not enable snappy compression support. This enables snappy unconditionally (because anyway it is not an external dependency)

[chklauser]

Lack of snappy compression is an issue for using rust-rdkafka in my organisation. While we deploy to linux, where we could use the mklove-based librdkafka-build, our developers mostly run Windows, where only the CMake-based librdkafka-build is available.

I'm not familiar at all with the mklove build system, but from what I can tell, the mklove system also enables snappy by default (hard-coded).

What would it take for this to get merged? Probably an entry in the changelog? I'm happy to help (obviously can't push to this PR directly)