Don't install header files unless told so

Change-Id: I9c1b88fe2501800de211ab5ac7f11910590bca46
Reviewed-on: http://review.couchbase.org/32435
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>