Don't install header files unless told so

Change-Id: I3e1139d2433b8c4a817e8f3e9e41e76c62444cfb
Reviewed-on: http://review.couchbase.org/32436
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>