AVRO-896. C: Snappy is an optional dependency

Buildbot doesn't have the snappy libraries installed, and it raised an error
trying to compile the C bindings with the previous AVRO-896 patch applied.
Turns out we weren't being careful enough when the Snappy library (and
presumably the other compression libraries) weren't installed.  Now this
correctly results in a good build, of a library with snappy support turned off,
as intended.


git-svn-id: https://svn.apache.org/repos/asf/avro/trunk@1507318 13f79535-47bb-0310-9956-ffa450edef68