Don't install header files unless told so

Change-Id: Ia6d9299350b40187b185e5a5adf7dfacb5b2dd47
Reviewed-on: http://review.couchbase.org/32434
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>