Warning that Apple support is disabled

I went to try out this library and got it linked and compiling, but couldn't figure out why my tests weren't registered or running. It took me a while of digging before I found the following.

It would have been really nice to have gotten a warning during compilation, rather than silently dropping the test, to know that what I was trying wasn't supported.

```
// Temporarily disable fuzz tests under MSVC/iOS/MacOS.
// They might not support all the C++17 features we are using right now.
// Disables all registration and disables running the domain expressions by
// using a ternary expression. The tail code (eg .WithDomains(...)) will not be
// executed.
#if defined(__APPLE__) || defined(_MSC_VER)
#undef FUZZ_TEST
...
```