What is the preferred method of allowing both bundled and system dependencies on older cmake versions?

Usage
1

What is the best/idiomatic way to allow users to either choose to use a dependency they already have, or to let cmake download it themselves? I’m thinking of doing something like the following, where I use externalproject if I want to use the bundled version and find_package if I want to use the external version:

option(USE_BUNDLED_UNIBILIUM "Use the bundled unibilium." ${USE_BUNDLED})

if(USE_BUNDLED_UNIBILIUM)
    ExternalProject_Add(unibilium
      URL ${UNIBILIUM_URL}
      ...
    )
else()
    find_package(unibilium 2.0 REQUIRED)
endif()

The idea is then that the Findunibilium.cmake finds all necessary libraries and directories and then defines an interface library named unibilium with those included. Then the main target can add unibilium as a dependency, which can either be from externalproject we built ourselves or from a system package.

This setup is the best I could come up with, but it feels unwieldy. I’m unsure if there’s a more sophisticated way to solve this. Any input is appreciated. My project has version 3.10 as the minimum required version.

2

[Moved explanation to top post]

3

Conclusion: cmake doesn’t seem to have the capabilities to solve this problem other than the above naive solution, which is disappointing as it’s a seemingly basic scenario.