Manually add /usr/local when Apple's python is being used.

Change-Id: I8dc5fab40cdaf048e46cef4ba7a33010a375ade6
Reviewed-on: http://review.couchbase.org/41282
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>