Build:
Follow NEURON build instructions for your OS. (precompiled for Linux x86_64 in repository provided), e. g.:
   1) ./configure --prefix=`pwd`
   2) make
   3) make install

Your libraries are in $ARCH/lib/ and your binaries in $ARCH/bin/ folders respectively.

Compile (preferably with gcc (shared - static is untested) - clang is untested):
g++  -L nrn-7.3/x86_64/lib/ nrn-7.3/src/oc/modlreg.o  example.cpp nrn-7.3/x86_64/lib/nvkludge.o -lnrnoc -lnrniv -livoc -loc -lneuron_gnu -lscopmath -lsparse13 -lsundials -lnrnmpi -lmemacs -lmeschach -livos -lreadline -lncurses -lm -locxt

Run (adjust path or compile/link statically above ./configure --enable-static and use g++ to link statically too!):
LD_LIBRARY_PATH=nrn-7.3/x86_64/lib/

See also the NEURON forum:
https://www.neuron.yale.edu/phpBB/posting.php?mode=reply&f=8&t=2638

See documentation for hoc (hoak):
http://www.neuron.yale.edu/neuron/static/new_doc/programming/hocsyntax.html
http://www.neuron.yale.edu/neuron/static/new_doc/modelspec/programmatic/topology/seclist.html
http://www.neuron.yale.edu/neuron/static/docs/help/neuron/neuron/classes/seclist.html#children

TODO:
Provide CMakeFile for UG plugin using precompiled NEURON libraries by step Build)