Add option to CLI based on CMake configuration - c++

Question

I have written a program prg which can be run using bash with some subcommands like so
$ prg subcommand1
Output of subcomman1
$ prg subcommand2
Output of subcomman2
The program is written in C++11 and compiled with CMake (3.10+).
Now, I'd like to add another optional command, say optional_subcommand.
Optional - in a sense that a user can decide whether to compile program with it when configuring Cake.
To clarify, when the code is not compiled withoptional_subcommand support, the output should be something along this lines
$ prg optional_subcommand
Option not supported...
The optional_subcommand requires external library to be installed. Hence find_package(SomeLib REQUIRED) must be invoked at some stage by CMake. The code for optional_subcommand requires this library to compile.
All I can think of is writing some dummy code for optional_subcommand which is then replaced on request by CMake. I think this can be achieved be asking CMake to overwrite, say optional_subcommand.cpp with either dummy or not.
Is proposed solution sensible or perhaps there is a better way to achieve this?

Answer 1

CMake can define a preprocessor symbol and add extra files to your target (below, prg) if needed:
if(SomeLib_FOUND)
target_compile_definitions(prg PUBLIC HAVE_SOMELIB)
target_sources(prg PRIVATE src/optional_subcommand.cpp)
endif()
You can then wrap the command dispatching code for optional_subcommand with #ifdef HAVE_SOMELIB.