Hi again,
It looks like the problem is with the debian/rules file. It sets PLATFORM=linux. When I change it to PLATFORM=hurd. SEARCH_EXE_PATH is in chicken-config.h as expected and dpkg-build -B succeeds. It seems to be portable if I set PLATFORM=$(shell dpkg-architecture -q DEB_TARGET_ARCH_OS) in debian/rules. I'll send a patch to the maintainer.