[build] Warning cleanup.

[ejgallego]

Still a long way to go, but we clean up a few warnings from the build.

[garrigue]

Thanks for trying. Some changes are good, some are less so:

• adding missing labels is probably a good thing, even if this can be a pain
• prefixing non-exported identifiers with _ seems wrong. I suppose they should actually be exported, otherwise what is the point of defining them ?
• I would prefer to keep warning 10 (non-unit statement) disabled. By design some functions return an ignorable result. I still don't see what is the benefit of ignoring it explicitly (at least inside lablgtk).
• If the self of object (self) is not used, then you should probably just remove the (self) altogether.