Allow for overrididing INSTALL_HEADER_FILES

Change-Id: If6aa9be1742521d58ef825d2356bace3a0694a2c
Reviewed-on: http://review.couchbase.org/32675
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>