[CMake] determining which system headerfile provides a given function

[René J.V. Bertin]

Hello,

I need to determine the availability of a certain function (reallocf) and which headerfile provides its prototype.
If there is a function that handles this case I must have overlooked it (including on google), so I tried repeating a check_symbol_exist() call repeatedly with the various known header file paths. But that also doesn't work, either because of caching or because I'm doing something else wrong.

So, how does one handle this kind of situation?

Thanks,
René