Build contrib in CI

#3306 (closed) is an example of build issue which we missed because we don't build the contrib/ directory in the CI, and we should.

This requires additional dependencies to the default CI image and code fixes (e.g. #3306 (closed), #2813 (closed)).

for d in contrib/dlz/modules/*; do
    [ -e $d/Makefile ] && make -C $d
done