Autoconf and CMake both compile small test programs to verify that the feature in question actually works. The test programs almost never actually do anything; the just refer to the feature of function that they rely on. If there is a compile or linker error then the feature isn’t available. In this case the compiler always outputs an error because the test program doesn’t have valid C syntax.

Of course in practice you shouldn’t be writing each test program by hand. Autoconf has macros that generate them; you would only need to supply an identifier or at most a single line of code and the rest would be created correctly. I’m sure CMake is similar in that regard, so the first red flag is that they wrote a whole test program.

That's exactly right. It was checking if the c code compiled to detect the landlock feature, and there was a single period in the middle of the code that made it always fail to compile and thus silently leave the feature disabled.