MB-12915: Use C++11 libstdc++ from cbdeps for Ubuntu 12.04

When running on Ubuntu 12.04 (which doesn't have a C++11 compliant
libstdc++ by default) install one from cbdeps.

Change-Id: Iccc4ebc18a195bd2fc083ee0da4fc6e3cb43e51b
Reviewed-on: http://review.couchbase.org/44484
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>