This is because the building process requires the pynq sdist to be built, which in turn builds the zcu104 bitstream as well as z1 and z2 bitstreams. I think you can use
make BOARDDIR=<your zcu111 boards dir> PYNQ_SDIST=<pynq.tar.gz>
pynq.tar.gz can be downloaded at: pynq · PyPI