Add BETTER_ENUMS_IGNORE_ATTRIBUTES_* for clang

[janten]

No description provided.

[janten]

The build errors in Travis CI seem to be unrelated to the change here. Could you check this, @aantron?

[aantron]

@janten Thanks. Yes, the Travis errors are unrelated, and we should ignore them :) I should probably disable Travis for now, as I don't have time to fix this the "right" way at the moment.

Can you say what error this PR addresses? These macros were added because a GCC version had broken handling of -Wattributes (#79). Is there a similar problem with some versions of Clang?

[janten]

I am using Xcode 11.3 and was getting warnings about BETTER_ENUMS_IGNORE_ATTRIBUTES_BEGIN being undefined, which I fixed here. A previous version of Better Enums, which did not have BETTER_ENUMS_IGNORE_ATTRIBUTES_BEGIN at all, also worked fine.

[aantron]

Thanks. I think these macros should be defined to expand to nothing on Clang. Can you try that?

[janten]

It works with empty #defines, too. I updated the source accordingly.

[aantron]

Thanks again!