Thanks, this helped a bit. Played with the order/recursiveness and found out a file in the SDK/usr/include has the same name as one header in my openssl include folder. I need to figure out how to still include the latter file, both in the Header Search Path and in the .h file, but pointing to the correct one?!

See here: https://stackoverflow.com/questions/79034494/xcode-16-error-use-of-undeclared-identifier-ctype-a And here: https://stackoverflow.com/questions/79034494/xcode-16-error-use-of-undeclared-identifier-ctype-a