Don't install header files unless told so

Change-Id: Icebca00ad1f179c0fa2d15c38e6cec2e85babb76
Reviewed-on: http://review.couchbase.org/32433
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>