Add `BUILD2_AUTOCONF_*` macros for Solaris and derivatives

We have a number of checks that detect Solaris, typically like so:

```
((defined(__sun) && defined(__SVR4)) || defined(__sun__))
```

I think it would be good to add the corresponding `BUILD2_AUTOCONF_*` macros similar to other platforms. It would be also good to understand how all this relates to derivatives (illumos, etc).