For now, the libraries can be built and installed using:
cd /usr/src/lib/libclang_rt/fuzzer && \
make obj && \
make depend && \
make && \
sudo make install
cd /usr/src/lib/libclang_rt/fuzzer_no_main && \
make obj && \
make depend && \
make && \
sudo make install
See https://llvm.org/docs/LibFuzzer.html for more information.