Don't install header files unless told so

Change-Id: Ib1db860ea4722d0a8091d0ae1dd7d004d9b67d57
Reviewed-on: http://review.couchbase.org/32432
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>