Turn on --enable-gcc-warnings-advisory by default

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