The fix seems to work for the package build that were broken with make 4.0, and so I think this report may be closed when that fix is released.

[Please retain the CC to -email is unavailable- so that updates to this report are also recorded in the Debian BTS]

Here's the minimal makefile needed to reproduce it, and a transcript:

,--- detect.mk ---
build-stamp:
echo $@

build-arch: build-stamp
`---

$ make --version | head -n1
GNU Make 3.81
$ make -f detect.mk -qn build-arch; echo $?
1

$ make --version | head -n1
GNU Make 4.0
$ make -f detect.mk -qn build-arch; echo $?
2

This is definitely a bug in GNU make 4.0 in handling -q.  The docs are clear on what the exit codes should be, and with -q make should exit with 1 if something needs to be updated and no error was detected.