Fix out of tree builds

Fix out of tree builds

Stop out of tree builds picking up the in-tree version and config.h

Change-Id: Icadc46cab66db72af2d475eac31b28d0ca10df90
Reviewed-on: https://code.wireshark.org/review/8204
Petri-Dish: Anders Broman <a.broman58@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>