Set variable CB_GNU_CXX11_OPTION with -std=gnu++11

Change-Id: I1cc0de7835afbd85be6b11cd5553059cf8f2348b
Reviewed-on: http://review.couchbase.org/43907
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Mark Woosey <mark.woosey@couchbase.com>