Remove default clang-specific warnings and add '-xc++'

[ibuclaw]

These aren't needed any more for the remainder sources.

-xc++ is required as:

clang: error: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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[wilzbach]

Travis is still broken, see e.g. #7505

[ibuclaw]

Hmm, apparently I messed something up in my initial local testing. As now I'm able to get those errors.

These global environment vars are really finicky to use. I'll have a look later...