provide macro to determine if compression is enabled

This avoids erroneous return codes when snappy support is not compiled
into libcouchbase

Change-Id: I1f05df43a6fe292a72044d875715a65f726feef1
Reviewed-on: http://review.couchbase.org/38502
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>