Set Python-related variables if they're not set

Set a variable we can test for if we have a relatively
modern python interpreter

Change-Id: Iddbd3d857d5ddcb36e3eae45e7ada96a64682bc8
Reviewed-on: http://review.couchbase.org/29291
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>