don't use sasl_client_done on OSX Yosemite

it looks like the provided libsasl2 on Yosemite isn't quite right (it
reports a sasl_version of 2.1.26, but doesn't provide all the symbols
that version should provide out of the actual dylib).  Conditionally
skipping calling that function for now on OS X.

Author: hanumantmk

src/mongoc/mongoc-sasl.c

@@ -170,7 +170,8 @@ _mongoc_sasl_destroy (mongoc_sasl_t *sasl)
 
 #if (SASL_VERSION_MAJOR >= 2) && \
     (SASL_VERSION_MINOR >= 1) && \
-    (SASL_VERSION_STEP >= 24)
+    (SASL_VERSION_STEP >= 24) && \
+    (!defined(__APPLE__))
    sasl_client_done ();
 #endif
 }