MB-22602: Use $KVEngineAPI to locate memcached headers; depend on xattr
MB-22602: Use $KVEngineAPI to locate memcached headers; depend on xattr

Use the KVEngineAPI_SOURCE_DIR/BINARY_DIR variable instead of hardcoding
'memcached/', and add the missing dependancy on xattr/visibility.h

Change-Id: I9e0b78f81816b09dc86d6924b2dfe495be6d3fe1
Reviewed-on: http://review.couchbase.org/78467
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>

couchbaselabs/couchstore,couchbaselabs/couchstore,couchbaselabs/couchstore