AVRO-896. C: Snappy compression codec.

Like all other codecs, this is optional.  It will only be activated if the
snappy C library is installed at build time.  Contributed by Grisha Trubetskoy.


git-svn-id: https://svn.apache.org/repos/asf/avro/trunk@1507309 13f79535-47bb-0310-9956-ffa450edef68