These are the steps I followed in order to program the RCX brick under Ubuntu 16.04 in C++. The documentation from http://brickos.sourceforge.net/docs/install-Debian.html is mostly still accurate with a few changes:

• The package “legos” is no longer available, but you can find it as “brickos” so you install it with

$ sudo apt-get install brickos

• The makefile  /usr/lib/brickos/Makefile.common used by the building process did not work for me out of the box. I had to comment out a couple of lines to make it work:

#ensureConfigured:
#     @if [ ! -f $(BRICKOS_ROOT)/.configured.flg ]; then \

• The c++ examples from /usr/share/doc/brickos/examples/demo/c++/ did not build, but removing the temperatureSensor.C file fixed the problem.

Hope this helps!