Don't install header files by default

Change-Id: Icd3fd51b6c4b656725444b6d418373617e38d298
Reviewed-on: http://review.couchbase.org/32544
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>