Selective warnings

[mpflanzer]

This patches allow to enable or disable specific warning, error, info or debug groups. At the moment the default is to suppress all messages. Maybe this should be changed to '-Wall' to preserve the old behaviour. Also there is no clear distinction between the different groups since right now all messages have been error messages. Messages that are generated by external calls are just reported as generic error without any specific subtype. But they can still be ignored all together.