fix: Make HAVE_CHALLENGE_NO_CHECK builds noisy at compile time

cmd_get_challenge.c uses `#ifdef HAVE_CHALLENGE_NO_CHECK` to (a) seed
the rolling challenge with a hardcoded constant and (b) skip the
verification of host-supplied challenges. The flag is meant for test
runs, but nothing in the source surfaced its presence to a release
build — a stray CHALLENGE_NO_CHECK=1 in a Makefile invocation would
ship a binary that accepts arbitrary challenge values without any
trace in the build log.

Emit a `#warning` whenever the macro is defined, so the build log
shows the bypass and CI can grep for it before promoting an artifact
to release.

(cherry picked from commit ea095ae8b4fc5461b3782f9bae3e52fa080fbfa3)