include: Fix -Wundef warnings in ansidecl.h

include: Fix -Wundef warnings in ansidecl.h

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++.