Make it possible to build builtin custom dissectors and GUI files with cmake too.

Change-Id: I28eee44f7c3a7e44da89f8b71cdacbe78115c7e3
Reviewed-on: https://code.wireshark.org/review/3943
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>