The problem is that the various `__has_foo` aren't actually reliable in practice - they don't tell you if the attribute, builtin, include, etc. actually works the way it's supposed to without bugs, or if it includes a particular feature (accepts a new optional argument, or allows new values for an existing argument, etc.).

    #if __has_attribute(cold)

You should use double underscores on attribute names to avoid conflicts with macros (user-defined macros beginning with double underscores are forbidden, as identifiers beginning with double underscores are reserved).

    #if __has_attribute(__cold__)
    #  warning "This works too"
    #endif

    static void __attribute__((__cold__))
    foo(void)
    {
        // This works too
    }

yep. C's really come a long way with the special operators for checking if attributes exist, if builtins exist, if headers exist, etc.

Covers a very large part of what is needed, making fewer and fewer things need to end up in configure scripts. I think most of what's left is checking for items (types, functions) existence and their shape, as you were doing :). I can dream about getting a nice special operator to check for fields/functions, would let us remove even more from configure time, but I suspect we won't because that requires type resolution and none of the existing special operators do that.