Abandon use of gcc attribute 'nonnull' from API
We use G_GNUC_WGET_NONNULL as a define for attribute nonnull for many API calls.
I thought and read about a bit more... It is more of a warning for callers like 'don't do that' with the bad side-effect that the compiler removes any parameter checks against NULL. This might be ok for internal / static functions for code optimization. But for an exported API should not use this when it has to check against NULL input values to exit gracefully. I might even be dangerous and introduce security flaws when the compiler silently removes explicit checks.