Added install target in GNUmakefile

Change-Id: I9fa4d3d7ef17cf918bea5747e8a3c3ae9a277392
Reviewed-on: http://review.couchbase.org/35437
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>