This quashes -Wundef warnings in ansidecl.h when compiled in C or C++.
In C, __cpp_constexpr and __cplusplus aren't defined so we evaluate
them to 0; conversely, __STDC_VERSION__ is not defined in C++.
This has caused grief when -Wundef is used with -Werror.
I've also tested -traditional-cpp.
include/ChangeLog:
* ansidecl.h: Check if __cplusplus is defined before checking
the value of __cpp_constexpr and __cplusplus. Don't check
__STDC_VERSION__ in C++.
34dbb5f346