Now that we depend on C++17, we can use these attributes that are part
of the language, instead of relying on macros to make them portable.
(GJS_USE still needs to be defined for public header files, which may be
compiled in C.)

Attributes which are new in C++17 are [[maybe_unused]], [[nodiscard]],
and [[fallthrough]].

[skip cpplint] because cpplint doesn't deal with C++ attributes:
https://github.com/google/styleguide/issues/165