I've had multiple patches fail due to missing the `--enable-gcc-warnings` flag in builds. I've also missed really important problems that these warnings would tell me about. Turning on `--enable-gcc-warnings` by default is probably not a good choice, because GCC warnings vary quite a lot between versions and `-Werror` will cause these new warnings to break builds. Without `-Werror`, there's no harm in enabling all of these warnings by default, and this makes us more likely to notice important warnings. We should do it. I suggest: 1. Make the behavior of `--enable-gcc-warnings-advisory` default when supported by the compiler 2. Add an `--enable-fatal-warnings` option to turn on `-Werror` 3. Alias `--enable-gcc-warnings` to `--enable-fatal-warnings` for backwards compatibility 4. Deprecate or ignore `--enable-gcc-warnings-advisory` 5. Optionally, we could have a `--disable-extra-warnings` flag, if there's a case where someone wants to turn off these warnings