This example is intended to be compiled with Codelite.
Codelite is a free cross-platform software...
If you want to compile it without codelite, you can do it with GNU make :
make -f clock-server-json.mk
You can then run the program that is in the Debug folder.
Nevertheless, I am not sure that it works on Windows™ and macOS ™, so you should follow my advice and use CodeLite...
This library provides a file for pkg-config that should also allow you to compile with gcc:
g++ -o clock-server-json $(pkg-config --cflags --libs modbuspp) main.cpp