build: improve check for ::(w)system

`AC_DEFINE()` takes `HAVE_STD__SYSTEM || HAVE_WSYSTEM` literally, meaning you
end up with the following in bitcoin-config.h:
```cpp
/* std::system or ::wsystem */
#define HAVE_SYSTEM HAVE_STD__SYSTEM || HAVE_WSYSTEM
```

This works for the preprocessor, because `HAVE_SYSTEM`, is defined, just unusually.
Remove this in favor of defining `HAVE_SYSTEM` to 1 in either case, given we
don't actually use `HAVE_STD__SYSTEM` or `HAVE_WSYSTEM`. We just use ::system if
we aren't building for Windows.

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