I'm going to guess that most of those issues were caused by implicit declarations no longer being valid.

    int main() {
        printf("Sans headers, this is valid C89.");
        return 0;
    }

Without an explicit declaration, C will consider this function to have the following signature based on the call site:

    int printf();

By the way, in C () doesn't mean "no parameters" it means "any parameters, which the compiler will infer from the call site and pass to the function."