Clang warnings: warning when using names reserved by C++ standard

[ovanes]

please remove from all names '__' (double underscore) or preceding underscore followed by a capital letter, e.g. '_Z'. These names are reserved for C++ standard library and C++ compiler intrinsics. Having such names in includes (especially guards) and using very restrictive compilation mode with clang results in a lot of compilation warnings.

[bluca]

Could you please send a PR to fix it?

[ovanes]

I'll check if I can overview everything...