Fix incompatibilities with c++2a mode in clang

[christinaa]

This closes #932.

[christinaa]

Minor correction, it was a fundamental type as-is but was gated behind an option. Since Clang 346892 it is now enabled by default.

This makes use of a rather simple feature test:

Builder.defineMacro("__cpp_char8_t", "201811L");

[vitaut]

Good catch, thanks!