We could relax the whole thing a bit, and maybe just make a plain
__cplusplus check, without a version number. This keeps the C side as
it always has been, but allows C++'s proper literals to kick in.
In addition, _Bool is also a major obstacle, as it does not exist in
C++ (and does so only for GCC due to some GNUisms in libstdc++ with
-std=c++98). Would you consider moving away from _Bool and replacing
all uses of it with bool? After all, <stdbool.h> by itself only
mandates the existence of bool/true/false.