/usr/bin/jackd and many other binaries in jack-audio-connection-kit-1.9.12-1.fc28 were not built with the full Fedora build flags. Specifically, the LDFLAGS injection is missing. More information about the build flags provided by redhat-rpm-config is available here: https://src.fedoraproject.org/rpms/redhat-rpm-config/blob/master/f/buildflags.md
fixed in rawhide