compilersupport: add support for C23's nullptr

It has the same semantics as C++11's `nullptr`.

Signed-off-by: Thiago Macieira <thiago.macieira@intel.com>

src/compilersupport_p.h

@@ -202,10 +202,14 @@
 
 #ifdef __cplusplus
 #  define CONST_CAST(t, v)  const_cast<t>(v)
-#  define CBOR_NULLPTR nullptr
 #else
 /* C-style const_cast without triggering a warning with -Wcast-qual */
 #  define CONST_CAST(t, v)  (t)(uintptr_t)(v)
+#endif
+
+#if defined(__cplusplus) || __STDC_VERSION__ >= 202311
+#  define CBOR_NULLPTR nullptr
+#else
 #  define CBOR_NULLPTR NULL
 #endif