Fixed check for -std=c++11

Change-Id: I7ed2f0d8f69e48afa894b68c51cd7d58f13bac2b
Reviewed-on: http://review.couchbase.org/30132
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>