How can I use "nullptr" for null pointers prior to C23?

In C23, the nullptr keyword got standardized. I would prefer to use nullptr instead of NULL prior to C23 too because it means that I could write code which compiles in:

C, prior to C23
C, since C23
C++

I could simply use NULL in both languages and every C standard, but that would be highly unusual in C++ code, and nullptr is becoming the norm in both languages anyway.

As far as I know, you are not allowed to use #define to replace keywords in the language, and that may cause problems when defining a compatibility macro. Basically, I need:

// only if this is neither C++ nor C23
#define nullptr /* something */

How can I properly define this macro?

Solution

Some things of note:

__STDC__ and __STDC_VERSION__ were added in "C95" (the addendum to ISO C 9899:1990).
You cannot check for nullptr using pre-processor conditionals because it is not a macro.
The user might forget to include stddef.h or equivalent header even when using a conforming C23 compiler so don't assume that this one is #included.
Current versions of gcc and clang support -std=c2x which does contain nullptr but sets __STDC_VERSION__ to a placeholder value 202000L.
Not all versions of C++ contain nullptr since it was added in C++11.

Therefore the macro checks should look something like this:

/* C++11 or later? */
#if (defined(__cplusplus) && __cplusplus >= 201103L)  
  #include <cstddef>

/* C2x/C23 or later? */
#elif ( defined(__STDC__) &&          \
        defined(__STDC_VERSION__) &&  \
        (__STDC_VERSION__ >= 202000L) )
  #include <stddef.h> /* nullptr_t */

/* pre C23, pre C++11 or non-standard */
#else
  #define nullptr (void*)0
  typedef void* nullptr_t;

#endif