cmake: use ${PROJECT_NAME}_IS_TOP_LEVEL instead of PROJECT_IS_TOP_LEVEL

[benedekkupper]

I use FetchContent_Declare() and FetchContent_MakeAvailable() to pull in the cib dependency to an ESP-IDF project (among others). Strangely, at least with cmake version 3.28.3, PROJECT_IS_TOP_LEVEL evaluates to true in such scenarios. The outcome of the mixup is a completely broken build. Using ${PROJECT_NAME}_IS_TOP_LEVEL solves the problem, as it produces the expected behavior.

[bdeane-intel]

Can you explain why this change fixes your problem?

The documentation for both is fairly identical:
https://cmake.org/cmake/help/latest/variable/PROJECT-NAME_IS_TOP_LEVEL.html
https://cmake.org/cmake/help/latest/variable/PROJECT_IS_TOP_LEVEL.html

And specifically says:
The variable value will be false in a directory added by FetchContent_MakeAvailable() if the fetched content contains a project() call.
...which is your case? So I do not understand the difference here.

Is it possibly caused by a problem somewhere else in the cmake you're running?
With the cache variables?
Is it a problem others will run into?
Is it changed by a policy? (e.g. https://cmake.org/cmake/help/latest/policy/CMP0180.html#policy:CMP0180)
Would it be better to fix the ${PROJECT_NAME} to cib here?