Don't install header files unless told so

Change-Id: Ia66c980a0dedebe6daa68242c6b2ad471bb28d72
Reviewed-on: http://review.couchbase.org/32431
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>