add way to make an include in a CMakePresets.json file optional

I am trying to get started contributing to a relatively large open source project. There is submodule in the project (community contributions) that is for all intents optional.

If one simply clones the primary repo, without submodules, cmake --list-presets is broken (file not found – with no details, btw). However, as I mentioned, there are presets within the primary repo that are perfectly reasonable to select that don’t require the submodule at all.

Please provide a way to mark an include as optional and if it is missing, do not error on cmake --list-presets.

Cc: @kyle.edwards