I’m not quite following what the diff here is suggesting - was this some cmake magic to detect if a feature was enabled, but the file had an intentional syntax error?
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.